What is the inverse square law for lighting?

For an ideal point light radiating freely, illuminance is inversely proportional to distance squared. The calculator uses E2 = E1 multiplied by the square of r1 divided by r2.

What happens to lux when lighting distance doubles?

The ideal illuminance falls to one quarter of the reference value. That is 25 percent of the light, or a decrease of two exposure stops.

How do I calculate distance for a target lux level?

Multiply the reference distance by the square root of reference lux divided by target lux.

How are lux ratios converted to exposure stops?

The stop change is the base-2 logarithm of the new lux divided by the reference lux. A ratio of one half is minus one stop and a ratio of one quarter is minus two stops.

Does the inverse square law work with softboxes and LED panels?

It becomes more accurate when the distance is large compared with the emitting surface. Close to a large soft source, the fixture is not point-like and actual falloff can differ substantially.

Why might a light meter disagree with the result?

Beam optics, source size, angle, reflections, diffusion, spill, fixture output changes, and meter placement can all make a real reading differ from the ideal model.

Are my lighting measurements uploaded?

No. Calculations, copying, and CSV generation happen locally in the browser.

Lighting Inverse Square Law Calculator — Distance, Lux & Exposure Stops

Predict lux at another distance or find the distance for a target lux level. See the ideal light ratio, percentage, and exposure-stop change instantly.

Reference light and target

Calculate

Distance unit

Reference illuminance (lux)

A meter reading at the reference distance.

Reference distance (m)

New distance (m)

Your measurements stay in this browser. Nothing is uploaded or stored.

Calculated lighting falloff

Illuminance at 2.00 m 250 lux From 1,000 lux measured at 1.00 m

Light remaining25.00%

Distance multiplier2.000×

Light change−2.00 stops

Compensation to match exposure+2.00 stops

What this means

Doubling the distance reduces ideal illuminance to one quarter: from 1,000 lux to 250 lux.

250 lux is approximately 23.23 foot-candles. Add 2.00 stops of camera exposure to maintain the same recorded brightness.

How to use the lighting inverse square law calculator

Take a reference reading. Measure lux at a known straight-line distance from the light's effective source or photometric reference point.
Choose the calculation. Predict lux at another distance, or solve for the distance that should produce a target lux level.
Keep the setup unchanged. Do not change fixture output, focus, zoom, diffusion, beam angle, or meter orientation between readings.
Use stops for exposure planning. A negative light change needs the same positive number of compensation stops to maintain the original camera exposure.
Confirm on set. Treat the answer as a starting estimate and verify the actual position with a light meter.

Inverse square law, distance, and exposure-stop formulas

For an ideal point source with unchanged output, illuminance E falls with the square of source-to-subject distance r:

E₂ = E₁ × (r₁ ÷ r₂)²

To solve for the distance that gives a target illuminance:

r₂ = r₁ × √(E₁ ÷ E₂)

The calculator converts the light ratio to photographic exposure stops with:

stop change = log₂(E₂ ÷ E₁)
compensation = −stop change

One stop is a doubling or halving of light. Moving twice as far away gives one quarter of the lux, so the change is −2 stops. Moving half as far away gives four times the lux, or +2 stops.

Formula references: OpenStax Physics — illuminance and the inverse square law and Nikon — exposure stops and doubling or halving light.

Quick lighting falloff reference

New distance	Light ratio	Light remaining	Stop change	Compensation
0.5× reference	4×	400%	+2.00 stops	−2.00 stops
0.707× reference	2×	200%	+1.00 stop	−1.00 stop
1× reference	1×	100%	0 stops	0 stops
1.414× reference	0.5×	50%	−1.00 stop	+1.00 stop
2× reference	0.25×	25%	−2.00 stops	+2.00 stops
4× reference	0.0625×	6.25%	−4.00 stops	+4.00 stops

When the inverse square model applies

The result is an ideal estimate. It assumes a point-like source, direct unobstructed light, fixed fixture output and beam shape, and the same meter orientation. It does not model reflections, flags, diffusion losses, atmospheric loss, beam-edge changes, dimmer curves, or fixture thermal behaviour.

Large softboxes, panels, windows, strip lights, and other extended sources are not point lights at close range. Their falloff can be slower and more complex until the measurement distance is large compared with the source. Fresnels, reflectors, lenses, zoom optics, and tightly controlled beams can also depart from ideal spherical spreading. For production exposure or lighting continuity, use this calculator for planning and confirm with a calibrated incident-light or lux meter.

Lighting inverse square law FAQ

What happens to lux when the distance doubles?

Ideal lux falls to one quarter. For example, 1,000 lux at 1 m becomes 250 lux at 2 m. That is a 75% reduction and a change of −2 exposure stops.

At what distance will the lux value be halved?

Multiply the original distance by the square root of two, approximately 1.414. If a light measures 1,000 lux at 2 m, it reaches an ideal 500 lux at about 2.83 m.

Why is doubling distance a two-stop loss?

Doubling distance produces one quarter of the illuminance. One half is −1 stop and one quarter is another halving, so the total change is −2 stops.

Is a stop the same as an f-stop?

A stop describes a factor-of-two exposure change. An f-stop is an aperture setting. You can compensate for a lighting change with aperture, shutter time, ISO, light output, or a combination, subject to the limits of the camera and fixture.

Can I use foot-candles instead of lux?

The inverse square ratio is the same in any illuminance unit. This calculator accepts lux and also displays the calculated result in foot-candles, using 1 foot-candle = 10.7639 lux.

Does this work for a softbox or LED panel?

Only as an approximation. It becomes more useful as the source gets small relative to its distance from the subject. Close to a large emitting surface, take actual meter readings.

Does this page upload my lighting data?

No. All calculations, copying, and CSV generation run locally in your browser.