Flash distance from guide number
Know where to stand before you shoot: a metric guide number of 36 at f/8 lights a subject correctly at 4.5 metres, in the same unit as the guide number on your flash chart. Free in your browser.
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- Your data stays on your device
Run — free
Worked example
- Guide number
- 36
- Aperture
- 8
- Distance
- 4.5
- F stop label
- f/8
- Precision
- 10
Photographers still need a fast answer to one practical question: at this f-stop, how far can my flash correctly light the subject? This flash distance from guide number calculator applies the classic direct-flash rule distance equals guide number divided by aperture f-number, using the manufacturer guide number you already know from the flash chart. Enter a positive guide number and the f-number you plan to shoot (for example eight for f/eight), and the engine returns the flash-to-subject distance in the same length unit as the guide number—metres for metric ratings, feet for imperial ones. Typical checks include a metric guide number of thirty-six at f/eight yielding four and a half metres, or a stronger unit at f/four reaching farther for group portraits. The path is pure finite arithmetic with no network calls, no sampling, and no date-dependent shortcuts.
What flash-to-subject distance from a guide number means
A flash guide number is a manufacturer rating that ties flash power, subject distance, and lens aperture under the simplifying assumption of a direct, unmodified flash on a subject of average reflectance. In the form this calculator uses, the correct flash-to-subject distance equals the guide number divided by the chosen f-number: distance equals guide number over aperture.
That is the inverse of the more common classroom statement that f-number equals guide number over distance; both express the same relationship, and this page is dedicated only to solving for distance when you already know the guide number and the aperture you want. Guide numbers are almost always published at a stated ISO—commonly ISO one hundred—and in either metres or feet. The length unit of the guide number must match how you interpret the answer, because the formula never converts between metric and imperial on its own.
A metric guide number of thirty-six at f/eight therefore means four and a half metres; an imperial guide number of one hundred twenty at f/eight means fifteen feet. There is no model of bounce ceilings, softboxes, zoom-head throw, high-speed sync, TTL preflashes, or ISO rescaling: those change real exposure and belong to metering or a broader guide-number tool.
Where flash distance from guide number earns its keep in the field and in software
On location, knowing how far the flash can reach at a chosen f-stop still gives a fast manual backup when TTL is unreliable, when you want a starting distance for a group portrait at f/eight or f/eleven, or when you are comparing two speedlights before buying. Studio assistants use the same arithmetic to place a bare flash so that the subject falls on a known working aperture without chimping every test frame.
Teachers and workshop leaders can generate answer keys for metric and imperial guide numbers without trusting opaque phone apps that silently switch units or hide the division. 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 subject distances, 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 an f-stop and the interface must show how far the light will correctly expose the subject under the direct-flash model. Pair it with f-number, hyperfocal, or related photography helpers when you are building a small lab of free, deterministic optics tools, 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 reach checks
From a flash GN and the f-stop you want, get the maximum correct subject distance before fine-tuning with a meter or histogram.
Studio placement planning
Place a bare flash so the subject sits at the distance implied by GN ÷ N for a target working aperture.
FAQ
What does the flash distance from guide number calculator cost?
It is free in your browser on this page.
What happens if aperture is zero?
Distance equals guide number divided by aperture, so zero f-number is undefined and not a valid photographic setting.
Are guide numbers in metres or feet?
Metric manuals use metres; some markets publish GN in feet.
Does this calculator adjust for ISO?
No. It uses the classic form distance = GN / N at the guide number you supply. If you need ISO scaling of effective GN, rescale the guide number first or use a broader guide-number tool.
Does the flash distance account for bounce or a softbox?
No. The rule assumes a direct, unmodified flash on a subject of average reflectance. Bounce ceilings, softboxes, zoom-head throw, high-speed sync and TTL preflashes all change the real exposure.
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/optics/flash-distance-from-guide-number \
-H "Authorization: Bearer $KIT_KEY" \
-H "Content-Type: application/json" \
-d '{"guide_number":36,"aperture":8}'const res = await fetch("https://api.kit.forhosting.com/optics/flash-distance-from-guide-number", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.KIT_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify({
"guide_number": 36,
"aperture": 8
})
});
const { task_id } = await res.json();import os, requests
res = requests.post(
"https://api.kit.forhosting.com/optics/flash-distance-from-guide-number",
headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
json={
"guide_number": 36,
"aperture": 8
},
)
task_id = res.json()["task_id"]<?php
$res = file_get_contents("https://api.kit.forhosting.com/optics/flash-distance-from-guide-number", false, stream_context_create([
"http" => [
"method" => "POST",
"header" => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
"content" => '{"guide_number":36,"aperture":8}',
],
]));
$task = json_decode($res, true);body := bytes.NewBufferString(`{"guide_number":36,"aperture":8}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/optics/flash-distance-from-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,
"aperture": 8
}Example response
{
"task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
"type": "optics.flash_distance_from_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.
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 flash-to-subject distance from a guide number means
This calculator accepts a positive guide number and a positive aperture f-number, optional decimal precision, and returns the rounded distance, the echoed inputs, an f-stop label such as f/eight, a short unit note, the method tag flash_distance_from_guide_number, the precision used, and a formula reminder string. Invalid inputs raise invalid_input, including missing fields, non-finite values, magnitudes above the supported ceiling, and any zero or non-positive guide number or aperture, because division by zero is undefined and a zero f-stop is not a physical aperture.
How inputs are read, how the division 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 subject distance. Both guide number and aperture must be strictly positive finite numbers. Zero aperture is always rejected, matching the product rule that guide-number arithmetic is undefined when the divisor is zero. Computation is a single division of guide number by aperture after validation; no square-root ISO scaling is applied here, so if your chart is rated at a different ISO than the one you shoot, rescale the guide number yourself before calling or use a tool that models effective guide number at working ISO. The preferred field names are guide_number for the rated guide number and aperture for the f-number N, but aliases such as gn and GN and guideNumber for the guide number, and N, f_number, fNumber, f_stop, and fstop for aperture are accepted so spreadsheets, query strings, and typed forms can reuse one contract. An optional precision field, with aliases decimals and dp, chooses how many decimal places to expose for 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 the classic GN over N division yourself.
Where flash distance from guide number earns its keep in the field and in 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 CI fixtures
Generate reproducible flash-to-subject distance answers for workshops, worksheets, and automated lighting calculator tests.
Technical questions
How do I calculate flash distance from guide number?
Send positive finite numbers guide_number and aperture (f-number N). The API returns distance = guide_number / aperture in the same length unit as the guide number.