Flash Guide Number Calculator
Calculates the correct aperture (or maximum distance) for a manual flash given its guide number and the flash-to-subject distance. Indispensable for manual flash photography without TTL metering.
Last updated: May 2026
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About this calculator
A flash unit's Guide Number (GN) encodes its maximum light output at a reference ISO (almost always ISO 100) and a specific zoom-head setting. The relationship between guide number, aperture, and flash-to-subject distance follows the simple formula GN = N × d, or rearranged, N = GN / d, where N is the aperture f-number and d is the flash-to-subject distance. This works because the inverse-square law of light dictates that doubling the distance quarters the illumination intensity, which requires opening two full stops. Variables: GN (rated at ISO 100, in either meters or feet), distance (in the same unit as GN). Edge cases: GN unit (m vs ft) must match the distance unit — mixing them produces a 3.28× error; ISO adjustment uses GN_effective = GN × √(ISO/100), so doubling ISO gives 1.41× more reach; zoom-head setting changes the effective GN (a flash zoomed to 105 mm has higher GN than at 24 mm because light is focused); bouncing off a ceiling or wall costs roughly 2 stops; large modifiers (softboxes, umbrellas) absorb and scatter light, often costing 1–3 stops; very dark subjects reflect less light and may require an extra stop. The formula assumes direct, unmodified flash at the rated zoom.
How to use
Example 1: Flash with GN = 40 (meters, ISO 100), subject at 5 m. Step 1: divide — 40 / 5 = f/8. Set your lens to f/8 for correct flash exposure at that distance. Verify: at 5 m the flash light has fallen off by the inverse-square law to a level that an f/8 aperture admits in the correct amount. Example 2: Same flash, subject moved to 2.5 m. Step 1: 40 / 2.5 = f/16. Verify: halving distance quadruples intensity (inverse square), requiring two stops smaller aperture — f/8 → f/11 → f/16 is exactly two stops. Conversely, if your lens is fixed at f/5.6, maximum reach = 40 / 5.6 ≈ 7.14 m; beyond that distance the flash cannot illuminate the subject correctly at ISO 100.
Frequently asked questions
What is a flash guide number and how is it used?
A guide number is a single value expressing a flash unit's power at a stated ISO and zoom setting, usually ISO 100 with the flash head at a standard zoom angle. It lets you calculate the correct aperture for any flash-to-subject distance using N = GN / d, or solve for distance using d = GN / N. Higher guide numbers indicate more powerful flashes capable of illuminating subjects at greater distances or with smaller apertures (more DoF). Manufacturers publish GN in either meters or feet — always confirm the unit before using it. GN is most useful with manual flash; TTL flash uses through-the-lens metering and adjusts power automatically.
How does ISO affect the effective guide number?
Guide numbers are calibrated at ISO 100. When you raise ISO, the sensor's amplified sensitivity effectively extends the flash's reach. The adjusted GN equals the rated GN multiplied by √(ISO / 100). At ISO 400 (four times ISO 100) the effective GN doubles because √4 = 2 — you can either use a smaller aperture for more depth of field or reach a subject twice as far at the same aperture. At ISO 1600 the effective GN quadruples. This is useful in dark venues or large rooms where flash alone would not reach the subject at base ISO. Conversely, dropping to ISO 50 cuts effective GN by √2 ≈ 0.71.
Why does distance have such a large effect on flash exposure?
Flash follows the inverse-square law: intensity falls off proportional to the square of the distance. Doubling the flash-to-subject distance reduces light landing on the subject to one quarter, requiring two full stops of compensation. The GN formula uses a linear distance but the resulting aperture changes by a square-root relationship — the math already accounts for the inverse-square falloff implicitly. In practical terms, moving a subject from 2 m to 4 m means you need to open up two full stops (e.g., from f/8 to f/4) to maintain the same exposure. This is also why a subject 1 m from the flash can be wildly overexposed if the meter expected 2 m.
What are common mistakes when using guide-number math?
Mixing meters and feet between GN and distance produces a 3.28× error in either direction. Forgetting the ISO adjustment leaves you stuck at the rated GN even when shooting at ISO 1600. Ignoring the zoom-head setting (most modern flashes auto-zoom with focal length) makes the effective GN drift significantly — a flash zoomed to 105 mm has a much higher GN than at 24 mm. Using the rated GN with a softbox or umbrella overstates output by 1–3 stops; modifiers absorb and scatter light. Finally, bouncing flash off a ceiling or wall typically costs roughly 2 stops, plus losses from the surface's reflectance — manufacturer GN figures assume direct flash.
When should I NOT use guide-number calculations?
TTL and automatic flash modes meter the actual scene and adjust power in real time, making manual GN math unnecessary (and often less accurate). Multiple-flash setups with different power levels and modifiers cannot be reduced to a single GN; use a flash meter or test exposures. Large modifiers like softboxes, umbrellas, beauty dishes, and grids change the effective output by 1–3 stops in ways the rated GN does not predict. Bouncing off ceilings or walls loses unpredictable amounts of light depending on color, distance, and reflectance. Studio strobes specified in watt-seconds (Joules) rather than guide numbers require a flash meter for accurate exposure. Finally, very near subjects (under ~1 m) often exceed the flash's minimum-distance specification and produce hotspots even at the smallest aperture.