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  30px 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.
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 }.
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.
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.
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.
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