Flash Guide Number Calculator
Use this flash guide number calculator to get manual flash exposure right first time: enter your flash's guide number and it finds the aperture, the flash-to-subject distance or the power setting you need at any ISO, allowing for diffusers, softboxes and bounce.
Enter your flash's guide number and work out the aperture, the distance, or the power setting you need.
Several flashes fired together into the same modifier act like one brighter flash.
How to Use the Flash Guide Number Calculator
Aperture, distance & power
- Enter your flash's guide number at ISO 100, in metres or feet
- Set your ISO and any modifier — diffuser, umbrella, softbox or bounce
- Choose what to work out and fill in the other two settings
- Read the result, then fine-tune with a test shot
Combine flashes
- Enter each flash's guide number in the same unit
- Read the combined guide number and how many stops it gains
How to Use a Flash Guide Number
A guide number is the flash-to-subject distance multiplied by the f-number for a correct exposure, at ISO 100 and full power. Because light spreads out with the square of distance, ISO and power change the guide number by their square roots.
The guide number formula
Guide number = distance × f-number
Aperture = guide number ÷ distance
Distance = guide number ÷ f-number
Effective GN = GN × √(ISO ÷ 100) × √(power)
Worked example: GN 45 at ISO 400 and half power, f/5.6
- Effective guide number: 45 × √4 × √0.5 = 63.6
- Flash-to-subject distance at f/5.6: 63.6 ÷ 5.6 = 11.4 m
- Through a softbox losing 1.5 stops, the same shot needs the flash at 6.8 m
Guide number chart
Effective guide number of a flash rated GN 60 (metres) at different ISOs and power settings. Divide by your aperture to get the flash-to-subject distance in metres — for example GN 42 at f/8 is 5.3 metres.
| ISO | 1/1 power | 1/2 power | 1/4 power | 1/8 power | 1/16 power |
|---|---|---|---|---|---|
| ISO 100 | 60 | 42 | 30 | 21 | 15 |
| ISO 200 | 85 | 60 | 42 | 30 | 21 |
| ISO 400 | 120 | 85 | 60 | 42 | 30 |
| ISO 800 | 170 | 120 | 85 | 60 | 42 |
| ISO 1600 | 240 | 170 | 120 | 85 | 60 |
| ISO 3200 | 339 | 240 | 170 | 120 | 85 |
Understanding Flash Exposure
Manual flash is predictable once you know how light falls off with distance.
Guide Number
Distance × f-number for a correct exposure at ISO 100 and full power. It sums up a flash's reach in one figure.
Example: GN 40 (m) → f/4 at 10 m, or f/8 at 5 m.
The Inverse Square Law
Light from a small source spreads over an area that grows with the square of distance. Double the distance and only a quarter of the light arrives — two stops less.
That's why guide numbers scale by square roots.
Power Settings
Each halving of power (1/1, 1/2, 1/4 …) is one stop less light. Lower power also recycles faster and gives a shorter, action-freezing flash.
ISO and Flash
Raising ISO makes the flash reach further: four times the ISO doubles the guide number. It also brightens the ambient light, changing the balance between flash and background.
Modifiers and Bounce
Diffusers, umbrellas and softboxes make light softer by making the source bigger — and cost light doing it. Bouncing off a ceiling can cost two to three stops.
Softer light, less reach: plan for the loss.
Aperture Controls Flash, Shutter Controls Ambient
Flash exposure depends on aperture, ISO, power and distance. Shutter speed (up to sync speed) only changes how bright the background ambient light records.
Tip: slow the shutter to brighten a dark background without changing the flash exposure.
Flash Guide Number FAQs
What is a guide number?
A measure of a flash's power: the distance at which it correctly exposes a subject, multiplied by the f-number, at ISO 100 and full power. A guide number of 60 (metres) means f/8 at 7.5 metres, or f/4 at 15 metres.
Why do guide numbers change with ISO and power?
Light spreads out with the square of distance, so guide numbers scale with square roots. Four times the ISO doubles the guide number; quarter power halves it.
Are manufacturers' guide numbers accurate?
They're usually quoted at the flash's longest zoom setting, which concentrates the light, so they can look generous. Check which zoom setting the rating applies to, and test your flash with a grey card at a known distance for a real-world figure.
How much light does bouncing or a softbox lose?
Typically 1 to 3 stops, depending on the modifier, the surface you bounce from and the room. The calculator's modifier list gives common starting points; a quick test shot tells you your exact figure.
How far will my flash reach?
Divide the guide number by the aperture. A flash with a guide number of 60 (metres) reaches 7.5 metres at f/8 and ISO 100; raise the ISO to 400 and it reaches 15 metres. Modifiers and bounce shorten that, so allow one to three stops.
Does shutter speed affect flash exposure?
Not for the flash itself — a flash burst is far shorter than normal shutter speeds. Shutter speed controls how much ambient light records, up to the camera's sync speed. Above it you need high-speed sync, which costs a lot of flash power.