diff --git a/README.md b/README.md
index 3eed79d..fc14f1c 100644
--- a/README.md
+++ b/README.md
@@ -1,7 +1,7 @@
 # Utilities Web App
 
 A collection of focused utilities implemented as a modern Vite.js + React Router application using the `app` folder, and the `framework` mode for routing from React Router with TypeScript and Tailwind CSS.
-Built with composable UI primitives (Radix + shadcn/ui), the project organizes converters, calculators and utilities
+Built with composable UI primitives (Base UI + shadcn/ui), the project organizes converters, calculators and utilities
 into reusable components and routes for easy extension.
 Provides PWA/service worker support and light/dark theming.
 
@@ -12,7 +12,7 @@ Demo: https://utilities-webapp.vercel.app/
 - **Framework**: Vite.js + React Router (framework mode) — v8.x (see `package.json`)
 - **Language**: TypeScript + React — React v19.x
 - **Styling**: Tailwind CSS
-- **UI primitives**: Radix UI + shadcn/ui
+- **UI primitives**: Base UI + shadcn/ui
 - **Package manager**: pnpm (recommended)
 
 ## 📁 What this repository contains
@@ -31,7 +31,7 @@ Demo: https://utilities-webapp.vercel.app/
   Roman Converter, Text to Speech, URL Slug Generator
 - **Programming Utilities**: Image to Base64
 - Light/dark theme switching (`next-themes`)
-- Responsive layout with accessible UI primitives (Radix + custom shadcn/ui components)
+- Responsive layout with accessible UI primitives (Base UI + custom shadcn/ui components)
 - Project built with modern tooling: Vite.js, React, TypeScript, Tailwind and ESLint/Prettier
 - Service Worker for offline support (experimental) and PWA features (workbox)
 
diff --git a/app/components/pages/calculators/date/date-add-subtract.tsx b/app/components/pages/calculators/date/date-add-subtract.tsx
index 415bca1..c3fb8d4 100644
--- a/app/components/pages/calculators/date/date-add-subtract.tsx
+++ b/app/components/pages/calculators/date/date-add-subtract.tsx
@@ -5,7 +5,7 @@ import { floor, random } from 'mathjs';
 import { useState } from 'react';
 import { Controller, Resolver, useForm } from 'react-hook-form';
 import { z } from 'zod/v4';
-import { Button } from '~/components/ui/button';
+import { Button, buttonVariants } from '~/components/ui/button';
 import { Calendar } from '~/components/ui/calendar';
 import { Card, CardContent, CardDescription, CardFooter, CardHeader, CardTitle } from '~/components/ui/card';
 import { Field, FieldContent, FieldError, FieldGroup, FieldLabel } from '~/components/ui/field';
@@ -116,23 +116,17 @@ export default function DateAddSubtract() {
                                     <Popover>
                                         <PopoverTrigger
                                             id="calendar"
-                                            render={
-                                                <Button
-                                                    variant={'outline'}
-                                                    className={cn(
-                                                        'pl-3 text-left font-normal max-sm:max-w-60 sm:w-full',
-                                                        !field.value && 'text-muted-foreground',
-                                                    )}
-                                                >
-                                                    {field.value ? (
-                                                        format(field.value, 'PPP')
-                                                    ) : (
-                                                        <span>Pick a date</span>
-                                                    )}
-                                                    <CalendarIcon className="ml-auto h-4 w-4 opacity-50" />
-                                                </Button>
-                                            }
-                                        />
+                                            className={buttonVariants({
+                                                variant: 'outline',
+                                                className: cn(
+                                                    'pl-3 text-left font-normal max-sm:max-w-60 sm:w-full',
+                                                    !field.value && 'text-muted-foreground',
+                                                ),
+                                            })}
+                                        >
+                                            {field.value ? format(field.value, 'PPP') : <span>Pick a date</span>}
+                                            <CalendarIcon className="ml-auto h-4 w-4 opacity-50" />
+                                        </PopoverTrigger>
 
                                         <PopoverContent className="w-auto p-0" align="start">
                                             <Calendar
diff --git a/app/components/pages/calculators/date/date-difference.tsx b/app/components/pages/calculators/date/date-difference.tsx
index 26ac3f0..33e884f 100644
--- a/app/components/pages/calculators/date/date-difference.tsx
+++ b/app/components/pages/calculators/date/date-difference.tsx
@@ -14,7 +14,7 @@ import { CalendarIcon } from 'lucide-react';
 import { Fragment, useMemo, useState } from 'react';
 import { Controller, useForm } from 'react-hook-form';
 import { z } from 'zod/v4';
-import { Button } from '~/components/ui/button';
+import { Button, buttonVariants } from '~/components/ui/button';
 import { Calendar } from '~/components/ui/calendar';
 import { Card, CardContent, CardDescription, CardFooter, CardHeader, CardTitle } from '~/components/ui/card';
 import { Field, FieldError, FieldLabel } from '~/components/ui/field';
@@ -191,26 +191,17 @@ export function DateDifference() {
                                         <Popover>
                                             <PopoverTrigger
                                                 id={`calendar-${key}`}
-                                                render={
-                                                    <Button
-                                                        variant="outline"
-                                                        className={cn(
-                                                            'w-full max-w-60 pl-3 text-left font-normal',
-                                                            !field.value && 'text-muted-foreground',
-                                                        )}
-                                                    >
-                                                        {field.value ? (
-                                                            format(field.value, 'PPP')
-                                                        ) : (
-                                                            <span>Pick a date</span>
-                                                        )}
-                                                        <CalendarIcon
-                                                            aria-hidden
-                                                            className="ml-auto size-4 opacity-50"
-                                                        />
-                                                    </Button>
-                                                }
-                                            />
+                                                className={buttonVariants({
+                                                    variant: 'outline',
+                                                    className: cn(
+                                                        'w-full max-w-60 pl-3 text-left font-normal',
+                                                        !field.value && 'text-muted-foreground',
+                                                    ),
+                                                })}
+                                            >
+                                                {field.value ? format(field.value, 'PPP') : <span>Pick a date</span>}
+                                                <CalendarIcon aria-hidden className="ml-auto size-4 opacity-50" />
+                                            </PopoverTrigger>
 
                                             <PopoverContent className="w-auto p-0" align="start">
                                                 <Calendar
diff --git a/app/components/pages/programming/base64-to-image.tsx b/app/components/pages/programming/base64-to-image.tsx
index 018de9c..b9b4683 100644
--- a/app/components/pages/programming/base64-to-image.tsx
+++ b/app/components/pages/programming/base64-to-image.tsx
@@ -3,7 +3,7 @@ import { Fragment, useState } from 'react';
 import { Controller, Resolver, useForm } from 'react-hook-form';
 import { Link } from 'react-router';
 import { z } from 'zod/v4';
-import { Button } from '~/components/ui/button';
+import { Button, buttonVariants } from '~/components/ui/button';
 import { Field, FieldContent, FieldError, FieldGroup, FieldLabel } from '~/components/ui/field';
 import { Input } from '~/components/ui/input';
 import { Select, SelectContent, SelectItem, SelectTrigger, SelectValue } from '~/components/ui/select';
@@ -131,18 +131,19 @@ export default function Base64ToImage() {
                             Reset
                         </Button>
 
-                        <Button
-                            type="button"
-                            variant="outline"
+                        <Link
+                            to={result ?? ''}
+                            download={fileName}
                             aria-label="Download Image"
-                            disabled={!result}
-                            nativeButton={false}
-                            render={
-                                <Link to={result ?? ''} download={fileName}>
-                                    Download
-                                </Link>
-                            }
-                        />
+                            aria-disabled={!result}
+                            tabIndex={!result ? -1 : undefined}
+                            className={buttonVariants({
+                                variant: 'outline',
+                                className: 'pointer-events-auto',
+                            })}
+                        >
+                            Download
+                        </Link>
                     </div>
                 </div>
             </form>
diff --git a/app/components/portal.tsx b/app/components/portal.tsx
index dece1d4..17fa206 100644
--- a/app/components/portal.tsx
+++ b/app/components/portal.tsx
@@ -1,10 +1,10 @@
-import { Slot as SlotPrimitive } from 'radix-ui';
+'use client';
+
+import { useRender } from '@base-ui/react/use-render';
 import * as React from 'react';
 import * as ReactDOM from 'react-dom';
 
-type SlotProps = React.ComponentProps<typeof SlotPrimitive.Slot>;
-
-interface PortalProps extends SlotProps {
+interface PortalProps extends useRender.ComponentProps<'div'> {
     container?: Element | DocumentFragment | null;
 }
 
@@ -21,15 +21,23 @@ function getServerSnapshot() {
 }
 
 function Portal(props: PortalProps) {
-    const { container: containerProp, ...portalProps } = props;
+    const { container: containerProp, render, ...portalProps } = props;
 
     const mounted = React.useSyncExternalStore(subscribe, getSnapshot, getServerSnapshot);
 
     const container = containerProp ?? (mounted ? globalThis.document?.body : null);
 
+    const element = useRender({
+        props: portalProps,
+        render,
+        state: {
+            slot: 'portal',
+        },
+    });
+
     if (!container) return null;
 
-    return ReactDOM.createPortal(<SlotPrimitive.Slot {...portalProps} />, container);
+    return ReactDOM.createPortal(element, container);
 }
 
 export { Portal };
diff --git a/app/components/ui/calendar.tsx b/app/components/ui/calendar.tsx
index 89cfa85..6057e71 100644
--- a/app/components/ui/calendar.tsx
+++ b/app/components/ui/calendar.tsx
@@ -16,7 +16,7 @@ function Calendar({
     components,
     ...props
 }: React.ComponentProps<typeof DayPicker> & {
-    buttonVariant?: React.ComponentProps<typeof Button>['variant'];
+    buttonVariant: React.ComponentProps<typeof Button>['variant'];
 }) {
     const defaultClassNames = getDefaultClassNames();
 
@@ -87,12 +87,12 @@ function Calendar({
                     defaultClassNames.day,
                 ),
                 range_start: cn(
-                    'relative isolate z-0 rounded-s-(--cell-radius) bg-muted after:absolute after:inset-y-0 after:end-0 after:w-4 after:bg-muted',
+                    'relative isolate z-0 rounded-s-(--cell-radius) bg-muted after:absolute after:inset-y-0 after:inset-e-0 after:w-4 after:bg-muted',
                     defaultClassNames.range_start,
                 ),
                 range_middle: cn('rounded-none', defaultClassNames.range_middle),
                 range_end: cn(
-                    'relative isolate z-0 rounded-e-(--cell-radius) bg-muted after:absolute after:inset-y-0 after:start-0 after:w-4 after:bg-muted',
+                    'relative isolate z-0 rounded-e-(--cell-radius) bg-muted after:absolute after:inset-y-0 after:inset-s-0 after:w-4 after:bg-muted',
                     defaultClassNames.range_end,
                 ),
                 today: cn(
diff --git a/app/routes/about.tsx b/app/routes/about.tsx
index 64d8b77..ca88429 100644
--- a/app/routes/about.tsx
+++ b/app/routes/about.tsx
@@ -28,7 +28,7 @@ export default function About() {
                         <h3 className="text-sm font-medium">Highlights</h3>
                         <ul className="mt-2 list-inside list-disc text-sm text-muted-foreground">
                             <li>Client-first utilities: most tools run entirely in the browser.</li>
-                            <li>Accessible UI primitives (Radix, Tailwind, shadcn patterns).</li>
+                            <li>Accessible UI primitives (Base UI, Tailwind, shadcn patterns).</li>
                             <li>Simple, composable components so new tools are quick to add.</li>
                         </ul>
                     </div>
diff --git a/tests/browser/components/calculators/date-add-subtract.browser.test.tsx b/tests/browser/components/calculators/date-add-subtract.browser.test.tsx
index ad8ec2e..a7f1698 100644
--- a/tests/browser/components/calculators/date-add-subtract.browser.test.tsx
+++ b/tests/browser/components/calculators/date-add-subtract.browser.test.tsx
@@ -2,7 +2,7 @@ import { describe, expect, test, vi } from 'vitest';
 import { render } from 'vitest-browser-react';
 import DateAddSubtract from '~/components/pages/calculators/date/date-add-subtract';
 
-// Radix Popover renders the calendar in a portal on document.body —
+// Base UI Popover renders the calendar in a portal on document.body —
 // we query document.body directly and use vi.waitFor to handle async opening/closing.
 async function openCalendarAndPickDay(triggerId = 'calendar', index = 0) {
     const trigger = document.getElementById(triggerId) as HTMLButtonElement | null;
@@ -18,9 +18,7 @@ async function openCalendarAndPickDay(triggerId = 'calendar', index = 0) {
         try {
             await vi.waitFor(
                 () => {
-                    openPopover = document.body.querySelector<HTMLElement>(
-                        '[data-slot="popover-content"][data-state="open"]',
-                    );
+                    openPopover = document.body.querySelector<HTMLElement>('[data-slot="popover-content"][data-open]');
 
                     if (!openPopover?.querySelector('[role="grid"]')) throw new Error('calendar not open');
                 },
@@ -38,7 +36,6 @@ async function openCalendarAndPickDay(triggerId = 'calendar', index = 0) {
     );
     buttons[index]!.click();
 
-    // Dismiss the popover with Escape (Radix listens on document)
     document.dispatchEvent(new KeyboardEvent('keydown', { key: 'Escape', bubbles: true, cancelable: true }));
 
     await vi.waitFor(() => {
@@ -48,7 +45,6 @@ async function openCalendarAndPickDay(triggerId = 'calendar', index = 0) {
     });
 }
 
-// Radix Select content is also a portal.
 async function selectAction(action: 'Add' | 'Subtract') {
     const trigger = document.getElementById('action') as HTMLButtonElement | null;
 
diff --git a/tests/browser/components/calculators/date-difference.browser.test.tsx b/tests/browser/components/calculators/date-difference.browser.test.tsx
index 7a56aa1..420e9b8 100644
--- a/tests/browser/components/calculators/date-difference.browser.test.tsx
+++ b/tests/browser/components/calculators/date-difference.browser.test.tsx
@@ -2,7 +2,7 @@ import { describe, expect, test, vi } from 'vitest';
 import { render } from 'vitest-browser-react';
 import { DateDifference } from '~/components/pages/calculators/date/date-difference';
 
-// Radix Popover renders the calendar in a portal on document.body —
+// Base UI Popover renders the calendar in a portal on document.body —
 // we query document.body directly and use vi.waitFor to handle async opening/closing.
 async function openCalendarAndPickDay(triggerId: string, index = 0) {
     const trigger = document.getElementById(triggerId) as HTMLButtonElement | null;
@@ -18,9 +18,7 @@ async function openCalendarAndPickDay(triggerId: string, index = 0) {
         try {
             await vi.waitFor(
                 () => {
-                    openPopover = document.body.querySelector<HTMLElement>(
-                        '[data-slot="popover-content"][data-state="open"]',
-                    );
+                    openPopover = document.body.querySelector<HTMLElement>('[data-slot="popover-content"][data-open]');
 
                     if (!openPopover?.querySelector('[role="grid"]')) throw new Error('calendar not open');
                 },
@@ -38,7 +36,6 @@ async function openCalendarAndPickDay(triggerId: string, index = 0) {
     );
     buttons[index]!.click();
 
-    // Dismiss the popover with Escape (Radix listens on document)
     document.dispatchEvent(new KeyboardEvent('keydown', { key: 'Escape', bubbles: true, cancelable: true }));
 
     await vi.waitFor(() => {
@@ -62,7 +59,7 @@ describe('DateDifference', () => {
             });
 
             const alerts = screen.container.querySelectorAll('[role="alert"]');
-            expect(alerts.length).toBe(2);
+            expect(alerts).toHaveLength(2);
             expect(alerts[0]?.textContent).toMatchInlineSnapshot('"A valid date is required."');
         });
     });