How to Use the Flash Guide Number Calculator
- Choose what to solve for. Use the Aperture / Distance / Guide Number toggle to pick the unknown you want the calculator to work out.
- Enter your flash’s rated guide number. Use the manufacturer’s GN at ISO 100, full power (usually printed in meters or feet).
- Set your ISO, power level, and number of flashes. Combining multiple identical flashes at the same distance and power increases your effective guide number.
- Account for modifiers. Pick a quick-pick chip for common modifiers (gel, diffuser, umbrella, softbox, bounce) or enter a custom stop value.
- Fill in the known value(s). Depending on the mode, enter your aperture and/or subject distance.
- Read the result instantly. The calculator updates live as you type — no button to press. Use the reference tables below to see how aperture changes across common distances and ISO settings.
Tip: Guide numbers are based on direct, bare flash. Use the modifier chips (or a custom stop value) to account for diffusers, softboxes, umbrellas, or bounce technique — real-world output is lower than the bare rating.
Guide Number / Aperture / Distance
Flash Guide Number Calculator
Solve for aperture, working distance, or effective guide number — factoring in ISO, power level, light-modifier loss, and multiple flashes. Built for direct on-camera or manual flash exposure.
What To Solve
Flash Setup
Results
Aperture By Distance
| Distance | Aperture |
|---|
At your current guide number, ISO, power, and modifier loss.
Aperture By ISO
| ISO | Eff. GN | Aperture |
|---|
At your current reference distance, power, and modifier loss.
How this works: guide number is a flash’s rated output at ISO 100, full power, direct (unmodified) light. Effective GN scales with the square root of ISO, the square root of power fraction, the square root of the number of identical flashes combined, and halves roughly every 2 stops of loss from diffusers, softboxes, or bounce surfaces. The core rule stays aperture = guide number / distance.
Understanding Flash Guide Numbers in Photography
Flash photography relies on balancing artificial light with your subject and scene. One of the most important specifications of any flash unit is its Guide Number (GN) — a simple way of describing the maximum power output of a flash. The guide number allows you to quickly calculate the correct aperture (f-stop) for a given subject distance and ISO setting.
The formula is straightforward:
Guide Number / Distance = Aperture (f-stop)
For example, if your flash has a guide number of GN 32 (ISO 100, meters) and your subject is 4 meters away, the correct aperture is f/8 (32 / 4 = 8). If you double the ISO from 100 to 200, the effective guide number increases by the square root of 2, giving you more exposure flexibility.
Real-world shooting rarely stops at ISO and distance. Power level, light modifiers, and multiple flashes all change your effective guide number too. Guide number scales with the square root of your power fraction (half power costs about 0.7 stops, not a full stop), roughly halves every 2 stops of loss from diffusers, softboxes, umbrellas, or bounce surfaces, and scales with the square root of the number of identical flashes you combine at the same distance and power.
The Flash Guide Number Calculator takes care of all of this math for you. Choose what you want to solve for — aperture, distance, or guide number — enter your flash setup, and the tool updates instantly as you type. The reference tables below show how your aperture changes across a range of distances and ISO settings at a glance.
This is especially useful for:
- Portrait photographers balancing flash power with natural light.
- Event and wedding photographers calculating flash exposure on the fly.
- Studio and off-camera shooters combining multiple flashes and ensuring consistent output across setups.
- Students and beginners learning how distance, ISO, and aperture interact in flash photography.
Keep in mind that manufacturer guide numbers assume direct flash at full power with no modifiers. Real-world use often includes softboxes, umbrellas, gels, bounce techniques, or multiple flashes combined, all of which change effective output. The calculator’s modifier chips and flash-count field let you account for these variables directly instead of guessing.
Whether you’re working with a single small speedlight or several strobes combined, the Flash Guide Number Calculator helps you achieve accurate, repeatable flash exposures without trial and error — saving time on set and ensuring professional results.
Related Tools & Guides
- Lighting Ratio Calculator — work out your key-to-fill ratio once you’ve got flash exposure dialed in.
- Inverse-Square Light Falloff Calculator — see how much exposure you gain or lose moving your flash closer or farther away.
- Speedlights Explained — a full guide to how speedlights work and when to reach for one.
- Studio Strobes Explained — how strobes differ from speedlights, and when the extra power is worth it.