Flash Power Matrix

What it does

Uses your flash's guide number (its power rating, specified by the manufacturer at ISO 100) to solve the relationship between aperture and subject distance for a correctly exposed flash shot — and scales that relationship for whatever ISO you're actually shooting at.

What you'll need

• Your flash's guide number at ISO 100 (check the flash's spec sheet — usually given in metres or feet; this calculator uses metres).
• Distance from flash to subject.
• Aperture (f-number).
• ISO you're shooting at.

Steps

1. Choose which of the three (aperture, distance, guide number) you want to solve for.
2. Enter the other two, plus your ISO.
3. Read the result.

Reading the result

Solving for guide number is useful the other way round: if you know the aperture and distance you need, it tells you the minimum flash power (guide number) required to reach that combination.

Tips

• Guide numbers only hold exactly in an open space with no light-bouncing surfaces — bounce flash, modifiers, and small rooms all change effective output, so treat this as a starting point and check your result with a test shot.
• Make sure your flash's spec sheet guide number and this calculator use the same distance unit — convert feet to metres (÷3.28) if needed before entering it here.

What is a flash guide number?

A guide number (GN) is a flash's power rating, set by the manufacturer at a standard reference point — ISO 100, in metres or feet. It's defined so that aperture × distance equals the guide number for a correctly exposed subject at that distance, at that ISO. It's essentially a flash's version of an EV rating: one number that captures how much light it can put out, comparable across different flash units.

Why photographers use it

Manual and off-camera flash work often happens without a light meter or reliable TTL metering — think adapted studio strobes, budget speedlights, or simply a photographer who prefers full manual control. Knowing the guide number relationship means you can work out how far a flash can reach at a given aperture, or what aperture to use at a known subject distance, entirely from the numbers on the flash's spec sheet — no test shots needed to get in the right ballpark.

Which guide number do I need?

For small on-camera fill flash in a well-lit room, GN20-30 (at ISO 100) is plenty. For bouncing flash off a ceiling or wall — which typically costs 1-2 stops of effective output — or lighting a subject several metres away, look for GN40-60 or higher. For outdoor daylight fill, where you're competing with the sun rather than a dim room, more power always helps. When comparing flashes across brands, check the test conditions behind the quoted number — some manufacturers rate guide number at the flash head's narrowest zoom setting, which inflates the figure well above what you'd get at a wider, more typical zoom.

Common mistakes

• Comparing guide numbers measured under different conditions. some manufacturers quote GN at the flash head's narrowest zoom (a tighter beam throws further), which is much higher than the same flash's wide-angle guide number. Always compare at the same zoom setting, ISO and distance unit.
• Mixing up feet and metres. a guide number given in feet is a meaningfully different figure from the same flash's metric rating. This calculator works in metres — convert a feet-rated spec (÷3.28) before entering it here.
• Assuming the guide number formula holds in every room. it's derived for a bare flash in open space with nothing nearby to bounce light off. Bounce flash, softboxes, diffusers, and small reflective rooms all change the effective output — sometimes substantially — from the open-space number.
• Expecting ISO to scale distance/aperture linearly. doubling ISO doesn't double your usable range — effective guide number scales with the square root of the ISO ratio, so a big ISO jump buys a comparatively modest reach increase. This calculator handles the scaling for you; the intuition alone is easy to get wrong.
• Buying a flash rated below what the math says you need. if solving for guide number says you need GN45 for your typical distance and aperture, don't buy something weaker expecting to compensate with ISO alone — you'll run out of headroom fast, especially outdoors against ambient light.

Example settings

FAQ

What does "guide number at ISO 100" mean if I always shoot at a different ISO?

It's just the manufacturer's reference rating. This calculator scales it to whatever ISO you enter automatically, using the same square-root relationship real flash output follows — you never need to adjust the rated guide number yourself.

Why does my flash seem weaker than its guide number suggests?

The guide number assumes a bare flash pointed directly at the subject in open space. Bounce flash, modifiers, diffusers, and small rooms with few reflective surfaces all reduce effective output below the open-space rating.

Does it matter whether I use feet or metres?

Yes — be consistent. This calculator works in metres throughout, so convert a feet-rated guide number (divide by roughly 3.28) before entering it.

Can I use this for multiple flashes or heavily bounced/modified light?

Not reliably — the guide number relationship assumes one flash aimed directly at the subject in open space. Multiple flashes, gelled/modified light, or heavy bounce need a light meter or test shots instead.

Is a higher guide number always better?

For reach and headroom, yes — but more powerful flashes are usually bigger, heavier, and slower to recycle between shots. Match the guide number to what your typical shooting distance and aperture actually require rather than maximising it.