Flash guide number calculator
A guide number of 36 at ISO 100 and a subject 4.5 metres away calls for f/8. Choose whether to solve for aperture or distance, set your ISO and get the effective guide number too. Free in your browser.
- Free
- Runs in your browser
- No sign-up
- Your data stays on your device
Run — free
Worked example
- Flash guide number
- 36
- Distance
- 4.5
- Working
- 100
- Rated iso
- 100
- Effective guide number
- 36
- Aperture
- 8
- Solved
- aperture
This flash guide number calculator turns a manufacturer guide number into a practical aperture or subject distance at the ISO you actually shoot. Photographers still use the classic relationship aperture equals effective guide number divided by distance, with the effective guide number scaled from the rated ISO by the square root of the ISO ratio. Enter a positive guide number, choose either distance or f-number, set working ISO and optional rated ISO, and the engine returns the missing value plus the scaled effective guide number. Typical checks include a metric GN of thirty-six at ISO one hundred and four and a half metres yielding f/8, and the same GN at ISO four hundred with f/8 yielding nine metres because the effective GN doubles. The path is pure finite arithmetic with no network calls, no sampling, and no date-dependent shortcuts.
What a flash guide number means and what this calculator returns
A flash guide number is a single manufacturer rating that links flash power, subject distance, and lens aperture for correct exposure under the simplifying assumption of a direct, unmodified flash on a subject of average reflectance. In the standard form used here, the f-number equals the effective guide number divided by distance, so a stronger flash or a closer subject allows a smaller aperture (larger f-number) for the same ISO.
Guide numbers are almost always published at ISO one hundred and in either metres or feet; the length unit of the guide number must match the unit you use for distance, because the formula never converts between metric and imperial on its own. When you change ISO, the effective guide number scales with the square root of the sensitivity ratio: doubling ISO multiplies effective GN by the square root of two, and two stops of ISO gain doubles the effective GN.
This calculator accepts a positive guide number, an optional rated ISO defaulting to one hundred, a working ISO defaulting to one hundred, and exactly one of distance or aperture. There is no model of bounce ceilings, softboxes, zoom-head throw, high-speed sync, or TTL preflashes: those change real exposure and must be handled by metering or experience.
Where guide-number math fits in field work, teaching, and software
On location, guide numbers still give a fast manual backup when TTL is unreliable, when you want a starting f-stop for a group portrait at a known distance, or when you are comparing two speedlights before buying. Studio assistants use the same arithmetic to place a bare flash at a distance that lands on f/eight or f/eleven at the working ISO without chimping every test frame.
Teachers and workshop leaders can generate answer keys for metric and imperial GNs without trusting opaque phone apps that silently switch units or forget ISO scaling. Developers call the same contract from scripts to seed deterministic test vectors for lighting planners, to normalize user-entered flash specs before a UI shows recommended apertures, or to satisfy programming-challenge statements that demand distance equals guide number over N.
Product and education tools use the solve when a learner types a guide number and a distance and the interface must show the f-stop and the effective GN after an ISO change. Pair it with hyperfocal or rule-of-five-hundred hobby tools when you are building a small lab of free, deterministic photography helpers, and remember that real scenes still need modifiers, bounce, and metering when the direct-flash assumption no longer holds.
What you can do with it
Manual flash starting points
From a flash GN and subject distance at your shooting ISO, get a starting f-number before fine-tuning with a meter or histogram.
ISO and power planning
Scale a rated GN to a higher ISO and solve for how far the flash can reach at a chosen aperture.
FAQ
What does the flash guide number calculator cost?
It is free in your browser on this page.
What happens if distance or aperture is zero?
Guide-number math divides by distance or aperture, so zero is undefined and not a valid photographic setting here.
Do I provide distance and aperture together?
No. Provide the guide number and exactly one of distance or aperture; the calculator returns the missing value.
Are guide numbers in metres or feet?
Metric manuals use metres; some markets publish GN in feet.
How does ISO change the flash guide number?
The effective guide number grows with the square root of the ISO ratio. Raising ISO to four times the rated value doubles it, so GN 36 at f/8 reaches 9 metres instead of 4.5.
Related tools
For developers — API access
Everything on this page is available programmatically. This section is for teams who want to wire it into their own systems; everyone else can just use the tool above.
API endpoint
Prefer to automate it? One authenticated POST creates the task; the result comes back by webhook or a signed link. The same capability also runs here on the web, by email and from Telegram — and soon from our app too.
Call it from your stack
curl -X POST https://api.kit.forhosting.com/hobby/flash-guide-number \
-H "Authorization: Bearer $KIT_KEY" \
-H "Content-Type: application/json" \
-d '{"guide_number":36,"distance":4.5}'const res = await fetch("https://api.kit.forhosting.com/hobby/flash-guide-number", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.KIT_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify({
"guide_number": 36,
"distance": 4.5
})
});
const { task_id } = await res.json();import os, requests
res = requests.post(
"https://api.kit.forhosting.com/hobby/flash-guide-number",
headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
json={
"guide_number": 36,
"distance": 4.5
},
)
task_id = res.json()["task_id"]<?php
$res = file_get_contents("https://api.kit.forhosting.com/hobby/flash-guide-number", false, stream_context_create([
"http" => [
"method" => "POST",
"header" => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
"content" => '{"guide_number":36,"distance":4.5}',
],
]));
$task = json_decode($res, true);body := bytes.NewBufferString(`{"guide_number":36,"distance":4.5}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/hobby/flash-guide-number", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)Example request
{
"guide_number": 36,
"distance": 4.5
}Example response
{
"task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
"type": "hobby.flash_guide_number",
"status": "queued",
"_links": {
"result": "/tasks/tsk_…/result"
}
}The API is asynchronous: the call returns a task_id immediately and the result arrives by webhook. Polling is capped at 1 req/s per task.
Pricing
Published price — no tokens, no invented credits. A failed task is never charged.
Limits
max_abs_value | 1000000000000000 |
Errors
| HTTP | Code | Meaning |
|---|---|---|
401 | unauthorized | Missing or invalid API key. |
402 | insufficient_balance | Your balance doesn't cover the task price. |
404 | unknown_type | That task type doesn't exist. |
429 | rate_limited | Too many requests. Use the webhook instead of polling. |
Technical notes
What a flash guide number means and what this calculator returns
It returns the rated guide number, both ISO values, the scaled effective guide number, the full distance and aperture pair, a solved tag naming which field was computed, a short unit note, the method tag flash_guide_number, the precision used, and a formula reminder string. Invalid inputs raise invalid_input, including missing guide number, both distance and aperture supplied together, neither distance nor aperture supplied, non-finite values, magnitudes above the documented ceiling, and any zero or non-positive distance or aperture, because division by zero is undefined and a zero f-stop is not a physical aperture.
How inputs are read, how ISO scaling works, and how results stay reproducible
Optional leading plus signs and scientific notation on numeric strings are allowed when they parse as finite decimals; embedded spaces, hex prefixes, and non-numeric tokens are rejected with a clear message so a bad cell does not become a wrong f-stop. Zero distance and zero aperture are always rejected, matching the product rule that guide-number arithmetic is undefined when either divisor is zero. Computation multiplies the rated guide number by that scale factor, then divides effective GN by the known distance or aperture. The preferred field names are guide_number for the rated GN, distance for subject distance, aperture for the f-number N, iso for working sensitivity, and rated_iso for the ISO printed on the flash chart, but aliases such as gn and GN for the guide number, d and dist for distance, N and f_number and f_stop for aperture, and gn_iso or guide_iso for rated ISO are accepted so spreadsheets, query strings, and typed forms can reuse one contract. Provide exactly one of distance or aperture: the missing value is the one solved. Working ISO and rated ISO default to one hundred when omitted, which matches most flash manuals; if you paste a GN that a manufacturer rated at a different ISO, set rated_iso explicitly so the scale factor sqrt of iso over rated_iso stays correct. An optional precision field, with aliases decimals and dp, chooses how many decimal places to expose for all numeric outputs, defaulting to ten and capped at fifteen so JSON goldens stay stable. Pair the free browser run on this page with the priced API when you need the identical mathematical result inside an automated pipeline without re-implementing ISO square-root scaling yourself.
Where guide-number math fits in field work, teaching, and software
On this page the work runs entirely in your browser; through the API each successful lookup costs $0.002, failed validation is not charged, and nothing is stored after the response returns. That split — free interactive exploration and a priced, identical API — matches the rest of the KIT tier-A catalog and lets you move from a classroom demo to an automated pipeline without changing the mathematical result.
Integration use cases
Teaching and test fixtures
Generate reproducible aperture and distance answers for workshops, worksheets, and CI assertions on lighting calculators.
Technical questions
How are aperture and distance related to guide number?
With effective GN at your working ISO, aperture = GN_eff / distance and distance = GN_eff / aperture. GN_eff = guide_number × √(iso / rated_iso).