CSS function · data last changed 10 Nov 2026
calc in email clients
Works in 8 of 21. Not supported in Outlook 365, Outlook (New), Outlook Classic and 7 more. Partial in Gmail Android and Gmail iOS. Untested in Superhuman.
Your reader in Outlook Classic sees the email without calc, even if it looks right where you tested it.
caniemail (MIT) via @emailens/engine 0.18.22
What to do instead in Outlook Classic
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.
There is no drop-in fix for calc in Outlook Classic. Design so the email still reads without it.
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calc is not among the 24 features we have screenshotted on real installs. The ones we have →
All 21 clients, problems first
| Client | Support |
|---|---|
| Outlook 365 | Not supported |
| Outlook (New) | Not supported |
| Outlook Classic | Not supported |
| Outlook iOS | Not supported |
| Outlook Android | Not supported |
| Outlook for Mac | Not supported |
| Yahoo Mail | Not supported |
| Yahoo Mail Android | Not supported |
| Yahoo Mail iOS | Not supported |
| AOL Mail | Not supported |
| Gmail Android | Partial See caveat 1 |
| Gmail iOS | Partial See caveat 1 |
| Superhuman | Untested |
Works in 8: Gmail, Apple Mail, Apple Mail iOS and 5 more. Show all
| Client | Support |
|---|---|
| Gmail | Works |
| Apple Mail | Works |
| Apple Mail iOS | Works |
| Samsung Mail | Works |
| Thunderbird | Works |
| HEY Mail | Works |
| Proton Mail | Works |
| Fastmail | Works |
Caveats (1)
- 1. Gmail Android, Gmail iOS: Partial. Not supported with non Google accounts.
Caveats as recorded by caniemail (MIT), one entry per wording.
How calc behaves
The calc() function performs runtime arithmetic on CSS values, allowing expressions like calc(100% - 40px). Email support is moderate, modern clients honor it, Outlook Classic does not, and behavior in Gmail varies across mobile and web. For maximum reliability, pre-compute values and supply literal pixel widths.
- Outlook Classic ignores calc() and falls back to the original CSS keyword (auto, etc.).
- Gmail supports calc() but mobile Gmail apps lag.
- Mixing units (e.g., 100% - 40px) is the most fragile pattern, same-unit math is more reliable.
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