About this tool

Retry-After Header Calculator

Build a spec-correct Retry-After header in delay-seconds or IMF-fixdate form, with an exponential backoff schedule for 429 and 503 responses.

The Retry-After Header Calculator turns a wait you want to impose into a Retry-After field value that conforms to RFC 9110 section 10.2.3, where the grammar is strictly an HTTP-date or delay-seconds = 1*DIGIT. It renders both forms side by side, formats the date as an IMF-fixdate in GMT as required for senders by RFC 9110 section 5.6.7, and builds the exponential backoff table behind a 429 or 503 so each successive rejection pushes the client further out. It also parses a value you already have and tells you whether a client would accept it.

How to use Retry-After Header Calculator

1. 1

   Open Retry-After Header Calculator on AltFTool — it loads instantly in your browser.

2. 2

   Choose "Wait a fixed amount" and enter "Wait" with a "Unit" of seconds, minutes, hours or days, or choose "Come back at a set time" and set "Retry at (your local time)".

3. 3

   Pick the "Response status", then fill "Base delay (seconds)", "Multiplier per attempt", "Cap (seconds)" and a "Jitter strategy" for the backoff schedule.

4. 4

   "Send this header" shows the delay-seconds form and the HTTP-date form side by side — press "Copy result", or paste an existing value into "Check a value you already have" to see how a client would read it.

Why use Retry-After Header Calculator

• Rejects malformed values

  Catches decimals, signs and unit suffixes that silently make the header unparseable.

• Correct date format

  Emits IMF-fixdate in GMT with fixed English day and month names, not a locale-dependent string.

• Backoff you can ship

  Shows the capped, jittered schedule so a retry storm spreads out instead of arriving in lockstep.

Common use cases

• An API team adding 429 responses to a rate limiter and needing the exact number of seconds until the token bucket refills, expressed as digits with no unit suffix.
• A platform engineer announcing a maintenance window that ends at 02:00 UTC and wanting the IMF-fixdate string to put on the 503 responses served by the holding page.
• Debugging a client that ignores your throttling, by pasting the value you send and confirming it is not something malformed like "120s" or "2 minutes".

Frequently asked questions