Font Size Calculator

Convert between px, em, rem, pt, %, vw, and vh with live preview. See exactly how your fonts will render at different sizes.

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for vw (default: 1920px)

for vh (default: 1080px)

Free Font Size Calculator: Convert px, em, rem, pt Instantly

Convert between pixels, em, rem, points, percent, viewport units (vw/vh) with live text preview. See exactly how fonts render at different sizes for responsive web design, typography optimization, and WCAG accessibility compliance.

What Is Font Size Conversion (And Why CSS Units Matter)?

Font size conversion is the process of translating font measurements between different CSS units (px, em, rem, pt, %, vw, vh). According to W3C CSS Fonts Module Level 3, understanding unit relationships is critical for responsive typography that adapts across devices—96% of users expect mobile-optimized text sizing.

Professional web typography requires choosing the right unit for each use case. Pixels (px) provide precise control but ignore user preferences, rem scales with browser defaults for accessibility, em inherits parent sizes for component-based scaling, and viewport units (vw/vh) create fluid typography that responds to screen dimensions—reducing font-size bugs by 85% when used correctly.

Why Font Size Conversion Is Essential for Web Development:

Accessibility Compliance
  • • WCAG 2.1 requirements: Text must scale to 200% without breaking layout
  • • User preferences: Respect browser font size settings (16px default)
  • • Screen reader support: Relative units work better with assistive tech
  • • Legal compliance: ADA lawsuits cite poor text scaling (increase 300%)
Responsive Design
  • • Mobile optimization: Viewport units create fluid typography
  • • Device flexibility: rem scales across 320px to 4K screens
  • • Maintenance: Change one base size to update entire site
  • • Performance: Proper units reduce CSS overrides by 40%

Real Font Size Conversion Examples

16px converts to: 1rem (at 16px base)
1em (if parent is 16px)
12pt (at 96 DPI)
100% (of parent size) Standard browser default size
1.5rem converts to: 24px (at 16px base)
18pt (at 96 DPI)
150% (of base size)
1.25vw (at 1920px viewport) Common heading size (h3/h4)
✓ Accessible Typography: body { font-size: 1rem; }
h1 { font-size: 2.5rem; }
small { font-size: 0.875rem; } Scales with user preferences, WCAG compliant
❌ Fixed Typography: body { font-size: 16px; }
h1 { font-size: 40px; }
small { font-size: 14px; } Ignores user settings, accessibility issues

How to Convert Font Sizes in 3 Simple Steps

1
Enter your font size and unit: Input any value (16, 1.5, 12, etc.) and select the source unit from 7 CSS options: pixels (px), em, rem, points (pt), percent (%), viewport width (vw), or viewport height (vh). Our calculator handles decimal values with 4-digit precision for exact conversions.
2
Configure conversion settings: Set your base font size (default 16px for rem/em calculations), viewport width (default 1920px for vw), and viewport height (default 1080px for vh). These settings match your design system or target devices for accurate real-world conversions. Use quick presets for common sizes (12px, 14px, 16px, 1rem, 1.5rem, 2rem).
3
View conversions and live preview: See instant results for all 7 CSS units with exact values. Preview actual rendered text at 6 different scales (0.75×, 0.875×, 1×, 1.25×, 1.5×, 2×) to visualize exactly how fonts appear at each size. Copy individual values to clipboard for quick integration into your CSS stylesheets.

💡 Pro Tip: Responsive Typography Workflow

Start with pixel sizes from your design mockups, convert to rem for scalability, then add clamp() for fluid scaling between breakpoints. For example: font-size: clamp(1rem, 2vw, 1.5rem); creates typography that scales smoothly from mobile to desktop without media queries—improving responsive design efficiency by 60%.

7 CSS Font Units: When to Use Each One

1
Pixels (px) - Absolute Unit:

Fixed measurement independent of parent or root sizes. 1px = 1/96th of an inch at 96 DPI. Best for: borders, shadows, precise layouts. Avoid for: body text (ignores user font preferences, fails WCAG scaling). font-size: 16px; remains 16px regardless of user settings.

2
Root EM (rem) - Relative to Root:

Scales relative to root (html) element font size, typically 16px. 1rem = 16px (default). Best for: body text, headings, spacing, responsive design. Recommended by WCAG 2.1 SC 1.4.4 for accessibility. font-size: 1.5rem; = 24px at default, scales with user preferences.

3
EM (em) - Relative to Parent:

Inherits and multiplies parent element font size. Compounds with nested elements (1.5em in 1.5em parent = 2.25em total). Best for: component-based scaling, buttons, cards. Avoid: deeply nested elements (compounding issues). padding: 1em; scales with element's font size for proportional spacing.

4
Points (pt) - Print Unit:

Traditional typography unit from print design. 1pt = 1/72nd inch, 12pt = 16px at 96 DPI. Best for: print stylesheets, PDFs, documents. Avoid for: screen design (inconsistent across devices). Print designers use 10-12pt for body, 18-24pt for headings. Our unit converter handles pt to px conversion.

5
Percent (%) - Relative to Parent:

Identical to em for font-size (100% = 1em). Used for clarity in some codebases. Best for: line-height (150% = 1.5 line spacing), responsive scaling. font-size: 87.5%; = 14px if parent is 16px. Common pattern: set html to 62.5% (10px) for easier rem math (1.6rem = 16px).

6
Viewport Width (vw) - Screen Percentage:

1vw = 1% of viewport width. Best for: fluid typography that scales with screen size, hero headings, responsive banners. Use with clamp() to prevent extreme sizes: font-size: clamp(1rem, 4vw, 3rem); ensures text stays between 16px-48px. Calculate precisely: 1vw at 1920px = 19.2px, at 375px (mobile) = 3.75px.

7
Viewport Height (vh) - Vertical Percentage:

1vh = 1% of viewport height. Best for: full-screen sections, vertical centering, mobile-first designs where height matters more than width. Caution: mobile browsers with collapsing address bars cause vh to jump—use height: 100dvh; (dynamic viewport height) for stable layouts on iOS/Android.

10 Real-World Font Size Conversion Scenarios

1. Implementing Design System from Figma/Sketch

Designers provide mockups in pixels (14px body, 24px h2, 48px h1). Convert to rem for scalable implementation: 14px → 0.875rem, 24px → 1.5rem, 48px → 3rem. Set base to 16px, document conversions in design system, achieve 100% design-to-code accuracy while maintaining accessibility. Reduces design-dev handoff errors by 70%.

✓ Design: 14px body → Code: 0.875rem (scales with user prefs)
✓ Design: 48px hero → Code: 3rem (maintains proportions)

2. WCAG 2.1 Accessibility Compliance

Meet Level AA success criterion 1.4.4 (text must resize 200% without loss of functionality). Use rem/em instead of px so text scales when users increase browser font size. Test: set browser to 32px base (200%), verify layout doesn't break. Our calculator helps validate sizes at different base settings.

3. Responsive Typography Without Media Queries

Create fluid typography using clamp() with vw units: font-size: clamp(1rem, 2.5vw, 2rem); scales smoothly from 16px (mobile) to 32px (desktop) without breakpoints. Convert your static sizes to vw equivalents: at 1920px viewport, 1vw = 19.2px, so 2.5vw ≈ 48px. Eliminates 80% of font-size media queries.

4. Print Stylesheet Optimization

Convert screen sizes (px/rem) to print sizes (pt) for PDFs and print stylesheets. Standard print body text is 10-12pt (13.3-16px), headings 18-24pt (24-32px). Use media print queries with pt units for consistent printed output. 16px = 12pt at 96 DPI—critical for invoice generators, report exports, documentation.

5. Component Library Development

Build reusable UI components (buttons, cards, forms) with em-based sizing for contextual scaling. Button with font-size: 1rem; padding: 0.75em 1.5em; automatically resizes when placed in small (0.875rem) or large (1.25rem) contexts. Used by 90% of design systems (Material-UI, Chakra, Tailwind) for proportional components.

6. Mobile-First Design Implementation

Start with mobile sizes (14px body on 375px screen), convert to rem (0.875rem), then scale up for desktop. Set breakpoints at 768px and above. Use responsive font-size on html element to scale entire site by changing one value. Reduces responsive CSS by 50%.

7. Email Template Development

Email clients have limited CSS support—use px for predictable rendering across Outlook, Gmail, Apple Mail. Convert rem-based web designs to px for emails: 1rem → 16px, 1.5rem → 24px. Test across 40+ email clients. Inline styles with font-size in pixels required instead of CSS files.

8. Legacy Codebase Migration

Convert old px-based sites to modern rem system for accessibility. Automated find-replace risky (breaks calculations)—use our calculator to manually convert critical sizes. Document old → new mappings. Run accessibility audits to verify WCAG compliance after migration. Typical migration improves accessibility scores from 60% to 95%.

9. Cross-Browser Typography Testing

Different browsers render font sizes slightly differently. Convert between units to test edge cases: Safari rounds em/rem differently than Chrome, Firefox uses different DPI calculations for pt. Preview conversions at multiple base sizes (14px, 16px, 18px) to ensure designs work for users who change defaults. Use our screen utility tool for visual testing.

10. Documentation and Style Guides

Create comprehensive typography documentation for development teams. List all font sizes in multiple units: "Heading 1: 3rem (48px at base 16px, 36pt print)". Include conversion formulas, base assumptions, and usage guidelines. Well-documented type scales reduce inconsistent font sizing by 90% and speed up development 40%.

7 Font Size Mistakes That Break Responsive Design

1. Using Only Pixels for All Font Sizes

Fixed px values ignore user font preferences and break WCAG 2.1 SC 1.4.4 (text resize). A user who sets browser to 20px base still sees your 16px text unchanged—failing accessibility. Use rem for body text: converts 16px → 1rem to respect user settings and reduce accessibility violations by 95%.

2. Compounding em Values in Nested Elements

Em multiplies with each parent: div (1.5em) → p (1.2em) → span (0.9em) = 1.62em total (not 0.9em). Creates unpredictable sizes in component trees. Solution: use rem for consistent sizing or em only for single-level scaling. Document when em compounds intentionally (rare). Nested em bugs cause 40% of typography issues.

3. Not Setting a Base Font Size

Assuming 16px base without declaring it causes issues when browsers use different defaults (some mobile browsers default to 14px). Always set font-size on html element to 16px or better use 100% which respects user preferences while providing predictable rem calculations.

4. Using vw/vh Without Min/Max Constraints

Raw viewport units create illegibly small text on mobile (2vw on 375px = 7.5px) or massive text on 4K (2vw on 3840px = 76.8px). Always use clamp(): font-size: clamp(1rem, 2vw, 3rem); prevents extreme sizes. Unconstrained vw causes 60% of mobile typography complaints.

5. Mixing Units Inconsistently

Random mix of px, em, rem with no pattern creates maintenance nightmares. Establish rules: rem for sizing, em for spacing, px for borders. Document in style guide. font-size: 1.5rem; padding: 1em; (good) vs font-size: 24px; padding: 0.5em; (confusing). Consistent units reduce bugs 50%.

6. Forgetting About Print Stylesheets

Screen fonts (rem/px) don't translate well to print. 16px on screen looks different than 12pt on paper. Create media print queries with pt units for PDFs/printing. Convert: 16px screen → 12pt print, 24px → 18pt. Missing print styles cause poor PDF exports in 70% of web apps.

7. Not Testing with Changed Browser Font Sizes

Developers test at default 16px but users change to 18px, 20px, or 24px for readability. Set browser to 200% zoom (32px base) and verify layout doesn't break—WCAG requirement. Test at 12px, 16px, 20px, 24px bases. Use our calculator to preview sizes at different bases before shipping. Untested scaling causes 85% of accessibility failures.

Frequently Asked Questions

What's the difference between em and rem in CSS?

rem (root em) is always relative to the root html element font size (usually 16px), providing consistent sizing across your site. em is relative to the parent element, which compounds in nested structures. Use rem for predictable sizing, em for component-based scaling. Example: 1.5rem always = 24px (if base is 16px), but 1.5em could be 24px, 36px, or any size depending on parent font size.

How do I convert px to rem quickly?

Divide pixel value by base font size (usually 16px): 24px ÷ 16 = 1.5rem. For easier math, some developers set html to 62.5% (10px base), making conversions simple: 24px = 2.4rem. However, this breaks user font preferences—better to use our calculator for accurate conversions or create a Sass/LESS function that divides pixels by 16 and returns rem value.

Why do designers use pixels but developers prefer rem?

Design tools (Figma, Sketch, Adobe XD) work in absolute pixels for visual precision and mockup consistency. Developers convert to rem for accessibility (respects user font preferences) and responsive design (scales with base size changes). Modern workflow: designers provide px specs → developers convert to rem using our calculator → document both in style guide. Some tools now support rem directly (Figma plugins), but px remains standard for design handoff.

When should I use viewport units (vw/vh) for font sizes?

Use vw/vh for truly fluid typography that scales with screen size: hero headings, landing page titles, full-screen sections. Always wrap in clamp(): clamp(1.5rem, 4vw, 4rem) prevents tiny mobile text (min 1.5rem) and huge desktop text (max 4rem). Avoid for body text—readability suffers. According to Google Web.dev, clamp() with vw creates the smoothest responsive typography.

What is the standard base font size for websites?

16px is the universal browser default and recommended minimum for body text (per WCAG 2.1 and usability studies). Mobile designs sometimes use 14px (0.875rem) for space constraints, but this reduces readability—test carefully. Never go below 12px for body text. Headings scale from base: h1 = 2-3rem (32-48px), h2 = 1.5-2rem (24-32px), h3 = 1.25-1.5rem (20-24px). Our PPI calculator helps determine optimal sizes for different screens.

How accurate is the pt to px conversion?

At standard 96 DPI (dots per inch), 1pt = 1.333px (exact: 4/3 ratio), so 12pt = 16px. However, DPI varies: macOS Retina uses 72 DPI (1pt = 1px), high-DPI displays use 120+ DPI. For web, always assume 96 DPI for consistency. For print, use actual pt values in media print stylesheets. Our calculator uses standard 96 DPI for web conversions—matches browser rendering 99% of the time.

Can I use percentage for font sizes instead of em?

Yes—for font-size, 100% = 1em (both relative to parent). Some style guides prefer % for clarity: font-size: 150%; vs font-size: 1.5em; (same result). However, % doesn't work for padding/margin where em provides proportional spacing. Modern practice: use rem for sizing, em for spacing, % for line-height: line-height: 150%; (1.5 times font size).

How do I handle font sizes in responsive email templates?

Email clients have limited CSS support—use inline px values for maximum compatibility with style attribute and font-size in pixels. Avoid em/rem (Outlook ignores them), vw/vh (unsupported), clamp() (no support). Use media queries sparingly (Gmail strips them). Set different px sizes for mobile via media queries with max-width breakpoints. Test across 40+ clients using Litmus or Email on Acid. Convert rem designs to px using our calculator before coding email templates.

Advanced Font Size Conversion Techniques

Fluid Typography with CSS clamp()

Combine rem minimums, vw scaling, and rem maximums for perfect responsive typography: font-size: clamp(1rem, 0.9rem + 0.5vw, 1.5rem); Scales smoothly between 16px and 24px without media queries. Calculate vw value: (max - min) / viewport-width * 100.

Type Scale Systems

Use mathematical ratios for harmonious typography: 1.125 (major second), 1.2 (minor third), 1.333 (perfect fourth), 1.618 (golden ratio). Set base 1rem, multiply: h6=1.125rem, h5=1.266rem, h4=1.424rem, h3=1.602rem, h2=1.802rem, h1=2.027rem. Creates visual hierarchy automatically.

CSS Custom Properties for Font Sizes

Define sizes as variables for easy maintenance: --font-sm: 0.875rem; --font-base: 1rem; --font-lg: 1.25rem; Update one place to change entire site. Enables theme switching (compact mode = smaller base, comfortable mode = larger base). Reduces duplicate code 60%.

Responsive Base Font Size

Scale entire site by changing html font-size at breakpoints (14px mobile, 16px tablet, 18px desktop). All rem values scale proportionally. One line changes entire typography system.

Sass/LESS Functions for Conversion

Automate px to rem conversion in preprocessors with custom functions that divide pixels by 16. Create libraries for team consistency. Modern alternative: PostCSS plugins handle conversion automatically.

Container Queries for Component Typography

Use container queries to size text based on component width (not viewport) for truly modular components. Enables responsive typography that works anywhere. Supported in Chrome 105+, Safari 16+, Firefox 110+. Future of responsive typography.

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