CSS Functions

calc

CSS function · 9 supported, 2 partial, 9 unsupported across 21 clients

The calc() function performs runtime arithmetic on CSS values, allowing expressions like calc(100% - 40px). Email support is moderate , modern clients honor it, Outlook on Windows does not, and behavior in Gmail varies across mobile and web. For maximum reliability, pre-compute values and supply literal pixel widths.

Client Support

Client	Category	Engine	Support
Gmail	webmail	Gmail Web	Supported
Gmail Android	mobile	Gmail Mobile	Partial
Gmail iOS	mobile	Gmail Mobile	Partial
Outlook 365	webmail	Outlook Web	Unsupported
Outlook (New)	desktop	Outlook Web	Unsupported
Outlook Classic	desktop	Microsoft Word	Unsupported
Outlook iOS	mobile	Outlook Mobile	Unsupported
Outlook Android	mobile	Outlook Mobile	Unsupported
Outlook for Mac	desktop	WebKit	Supported
Apple Mail	desktop	WebKit	Supported
Apple Mail iOS	mobile	WebKit	Supported
Yahoo Mail	webmail	Yahoo	Unsupported
Yahoo Mail Android	mobile	Yahoo	Unsupported
Yahoo Mail iOS	mobile	Yahoo	Unsupported
Samsung Mail	mobile	Samsung	Supported
Thunderbird	desktop	Gecko	Supported
HEY Mail	webmail	WebKit	Supported
Proton Mail	webmail	Proton	Supported
AOL Mail	webmail	AOL	Unsupported
Fastmail	webmail	Fastmail	Supported
Superhuman	desktop	Blink	Unknown

Client-by-client behaviour for calc

Fully supports calc (9): Gmail, Outlook for Mac, Apple Mail, Apple Mail iOS, Samsung Mail, Thunderbird, HEY Mail, Proton Mail, Fastmail.

Partial support (2): Gmail Android, Gmail iOS. Expect rendering quirks unique to each engine. Partial support typically means a subset of values, an ignored shorthand, or sanitizer-specific rewrites.

No support (9): Outlook 365, Outlook (New), Outlook Classic, Outlook iOS, Outlook Android, Yahoo Mail, Yahoo Mail Android, Yahoo Mail iOS, AOL Mail. Plan fallbacks for these clients before relying on calc in production sends.

Behaviour unverified in: Superhuman.

When to use calc in email

Sizing a side-bar column relative to a fluid main column.
Reserving pixel-perfect padding inside a percentage-width container.
Calculating button widths that respect their padding without breaking on mobile.

Rendering behaviour and edge cases

Outlook on Windows ignores calc() and falls back to the original CSS keyword (auto, etc.).
Gmail Web supports calc() but mobile Gmail apps lag.
Mixing units (e.g., 100% - 40px) is the most fragile pattern , same-unit math is more reliable.

Recommended fallback strategy

Provide a literal width fallback before any calc() declaration so Outlook receives a sensible value. Avoid calc() for critical layout dimensions unless every targeted client supports it.