Flash Guide Number Calculator

1. Enter your flash guide number in metres, as quoted at ISO 100.
2. Enter the aperture you are shooting at.
3. Enter your ISO, which extends the range at higher values.
4. Read the distance at which the flash correctly exposes the subject.

A guide number is a measure of flash power that ties together aperture and distance. Divide the guide number by the aperture and you get the distance the flash correctly exposes; divide it by the distance and you get the aperture to use.

Guide numbers are quoted at ISO 100, usually in metres, and sometimes in feet. A bigger guide number means a more powerful flash that reaches further or allows a smaller aperture at the same distance. It is the one number that lets you compare two very different flashes at a glance, and the starting point for setting a manual flash correctly without a light meter in hand.

1. Adjust the guide number for ISO by multiplying by the square root of ISO over 100.
2. Divide the adjusted guide number by the aperture for the working distance.
3. To find the aperture instead, divide the guide number by the distance.
4. Remember light falls off with the square of distance, the inverse square law.

Because light spreads out, doubling the distance needs four times the power, or two stops wider aperture, to keep the same exposure. This steep fall-off is the reason flash-lit subjects can look bright while a background just a few metres behind them falls into darkness, an effect photographers use deliberately to separate a subject from its surroundings.

A speedlight has a guide number of 36 in metres at ISO 100. At f/5.6 the working distance is 36 divided by 5.6, about 6.4 metres. Raise the ISO to 400, two stops, and the guide number effectively doubles to 72, so the same f/5.6 now reaches about 12.8 metres. Stop down to f/8 at ISO 100 and the range drops to about 4.5 metres. This is why raising ISO is such a powerful lever for flash photographers working at a distance: a couple of stops of ISO can more than double how far a modest flash will throw useful light, turning a close-range unit into one that reaches across a room.

Guide Number

A more powerful flash has a higher guide number and reaches further at any aperture.

Aperture

A wider aperture, a smaller f-number, lets the flash reach further; stopping down cuts the range.

ISO

Higher ISO extends the range, scaling the guide number by the square root of the ISO ratio.

Zoom and Modifiers

Zooming the flash head or removing a diffuser raises the effective guide number; softboxes lower it.

Working distance at ISO 100 for a guide number of 36.

Each stop down roughly cuts the range by a factor of 1.4, because distance scales with the square root of the light.

Manual Flash

Set the aperture for a subject at a known distance without metering.

Checking Reach

See how far a flash will correctly light a scene at your settings.

Comparing Flashes

Judge one flash against another by its guide number.

1. Mixing Metres and Feet

Guide numbers come in both; make sure yours matches the distance units.

2. Ignoring ISO

The quoted guide number is for ISO 100; higher ISO extends the range considerably.

3. Forgetting Inverse Square

Doubling the distance needs four times the power, not twice.

4. Trusting the Headline Number

Makers quote the guide number at full zoom; real-world reach with a diffuser is lower.

The guide-number maths is exact, and the ISO scaling is standard. Real flash output varies with zoom, modifiers and reflective surroundings.

It is a measure of flash power that links aperture and distance. Distance equals the guide number divided by the aperture at ISO 100.

Divide the guide number by the aperture. A guide number of 36 at f/5.6 gives a working distance of about 6.4 metres.

Higher ISO extends the range. The effective guide number scales with the square root of ISO over 100, so ISO 400 doubles it.

Divide the guide number by the distance. A guide number of 36 for a subject 6 metres away needs about f/6.

Because of the inverse square law: light spreads out, so intensity falls with the square of distance.

Both are used. Metres is common in specifications; always match the guide number units to your distance units.

Yes. A diffuser or softbox spreads and softens the light, lowering the effective guide number and the reach.

Makers quote it at maximum zoom and full power, so real reach with a wide zoom or diffuser is lower.

No. The calculator runs entirely in your browser and nothing you enter is stored or sent anywhere.

• Guide number (Wikipedia).
• Flash (photography) (Wikipedia).
• Inverse-square law (Wikipedia).