40px → REM

40px to REM - 2.5rem at 16px Base

At the default browser root size of 16px, 40px equals 2.5rem. Formula: rem = px ÷ 16.

Last reviewed: June 28, 2026

CSS snippet

.example {
  font-size: 2.5rem; /* 40px at 16px root */
  padding: 2.5rem;
}

PX to REM formula

rem = px ÷ root font size

40px ÷ 16px = 2.5rem

Worked example: To convert 40px at a 16px root, divide 40 by 16. 40 ÷ 16 = 2.5. Write font-size: 2.5rem; instead of font-size: 40px; so text scales when users enlarge their browser default font size.

Common use cases at 40px (2.5rem)

Category	Tailwind / design use	CSS property example
Font size	text-4xl (2.5rem)	`font-size: 2.5rem;`
Spacing	p-10	`padding: 2.5rem;`
Gap / margin	p-10	`gap: 2.5rem;`

  
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Also see  36px to rem
 
and  48px to rem

.
 
 
 40px to REM FAQ
How do you convert 40px to rem?
Divide 40 by the root font size. At the default 16px base: 40 ÷ 16 = 2.5rem. Formula: rem = px ÷ root font size. Paste font-size: 2.5rem; into your CSS so typography scales when users change their browser default font size.
What is 40px in rem at 16px base?
At a 16px root font size, 40px equals exactly 2.5rem. This is the browser default unless your project or the user changes html { font-size }. If your root differs, scale with rem = 40 ÷ yourRootPx before shipping production styles.
Should I use 2.5rem or 40px in CSS?
Use 2.5rem for typography and spacing that should scale with the user's root font-size preference - important for accessibility. Reserve 40px for fixed values like 1px borders or bitmap image dimensions. Prefer rem for layout rhythm so zoom and OS font settings do not clip text.
How do I convert 2.5rem back to pixels?
Multiply 2.5rem by 16: 2.5 × 16 = 40px. Reverse formula: px = rem × root font size. Use that when a design tool exports rem tokens and you need pixel sizes for image assets, canvas drawing, or developer handoff docs.
Where is 40px commonly used in web design?
40px (2.5rem) maps to text-4xl (2.5rem) in typical design systems. For Tailwind spacing, see p-10. Check the use-case table on this page for CSS property examples covering font-size, padding, and gap so you can pick a consistent token across components and breakpoints.
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PX to REM Converter - any value, custom base
  
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PX to REM cheat sheet - 12px through 64px