ND Filter Calculator

Convert between the ND factor (such as ND8), the optical density (such as 0.9), the number of stops and the transmittance. You can also find the exposure time with an ND filter on.

What to calculate

Quantity to enter

ND factor (1 or more)

Decimal places for stops and density

Converted values —

Breakdown
Original exposure time	—
Target exposure time	—
Exposure time with ND	—
ND factor	—
Optical density	—
Stops	—
Transmittance	—

The marked value on a product (ND8 = 0.9 and so on) is a rounded value or a name. It can differ a little from the theoretical value (the theoretical density of ND8 is 0.903, and ND1000 is about 9.97 stops). ND names differ by maker and use, and no public standard for them was found.

The exposure-time conversion is an ideal calculation. Reciprocity failure in long exposures (sensitivity drops as the exposure gets longer) and sensor noise are not included.

The number copied is the large value above (stops and density at the number of decimal places you chose).

Common ND filters (theoretical values)
ND	Stops	Density	Transmittance (%)

The four quantities

ND factor (the 8 in ND8): by what factor the light is reduced. ND8 lets through 1/8 of the light.
Optical density (such as 0.9): the base-10 logarithm of the factor. Transmittance = 10 to the power of (−density).
Stops (such as 3): the base-2 logarithm of the factor. One stop halves the light.
Transmittance (%): the share of light that passes. It is the reciprocal of the factor (a factor of 8 is 12.5%).

The conversions: factor F = 2^stops = 10^density = 100 ÷ transmittance (%), and density = stops × log10(2) (about 0.30103 per stop).

Why ND names can differ

ND8 = 3 stops (exact). The factor 8 is 2³. The theoretical optical density is 0.903, so a product marked "ND0.9" is using a rounded 0.9.
A density of exactly 0.9 gives a factor of 10^0.9 = about 7.943, or about 2.990 stops. That is slightly different from ND8 (3 stops).
ND1000 = a factor of 1,000 = about 9.966 stops (log2 of 1000). It is sometimes called "10 stops", but 10 stops is 2¹⁰ = 1024. A filter sold as "ND1000" may be a factor of exactly 1,000 (density 3.0) or the 10-stop filter (1,024), so check which one it is.
ND2 is 1 stop and ND4 is 2 stops. The number in the name is the factor, not the number of stops. ND16 is 4 stops, not 16.
Different makers use different notations (a factor such as ND8, a density such as ND0.9, or a number of stops), so check which one a product's name refers to.

Exposure time with an ND filter

An ND filter reduces light, so you can lengthen the exposure by the same factor to keep the same brightness. New exposure time = original exposure time × ND factor. Conversely, the ND factor needed = target exposure time ÷ original exposure time. If the target is shorter than the original, an ND filter cannot do it.

Enter the exposure time as a fraction such as 1/60, or as a number of seconds (more than 0, up to 100,000 seconds).

Examples

ND8 is 3 stops, an optical density of about 0.90 and a transmittance of 12.5%.
ND1000 is about 9.97 stops (9.966), a density of 3.00 and a transmittance of 0.1%.
ND1024 is 10 stops, a density of about 3.01 and a transmittance of about 0.0977%.
A density of 0.9 gives a factor of about 7.94, about 2.99 stops and a transmittance of about 12.59%.
3 stops is ND8. A transmittance of 25% is ND4 (2 stops).
1/60 s with ND1000 gives about 16.67 s (1/60 × 1000). 1/60 s with 10 stops (ND1024) gives about 17.07 s.
Going from 1/60 s to 30 s needs a factor of 1,800 (about 10.81 stops, a density of about 3.26).

Stacking filters

When you stack filters, density and stops add, and ND factors multiply. For example, ND8 plus ND4 is 3 + 2 = 5 stops, or a factor of 8 × 4 = 32. This tool has no stacking mode: enter the combined value.

Notes

The real transmittance of an ND filter can differ from its marked value because of the product, the wavelength and any color cast.
The exposure conversion is an ideal one. It does not include reciprocity failure (lower sensitivity in long exposures) or sensor noise.
Ranges: factor 1 to 1,000,000,000, density 0 to 9, stops 0 to 30, transmittance more than 0 and up to 100. A factor of 1 means no ND.
For the angle of view, see the Field of View Calculator, and for equivalent focal lengths, the Crop Factor Calculator.

How it is calculated, and its precision

When the factor is a whole number or a fraction, the factor and the transmittance are exact fractions. A factor that is a whole power of 2 (ND2 to ND1024 and so on) gives exact stops, and a factor that is a whole power of 10 gives exact density. Other values use floating-point log2, log10 and powers, trimmed to 12 significant digits and then rounded to the number of decimal places you chose (marked "rounded").

Sources

ShutterMuse, "Understanding Neutral Density Filter Names and Numbers": https://shuttermuse.com/neutral-density-filter-numbers-names . Its table was read on 2026-11-01 and agrees with the values above (ND8 = 3 stops, density 0.9, 12.5%; 1024, "sometimes called ND1000"). It is an explanatory article, not a standard.

Frequently asked questions

How many stops is ND8?

3 stops. The factor 8 is 2³, so the light is reduced to 1/8, which is 3 stops.

Is ND1000 10 stops?

A factor of 1,000 is about 9.97 stops in theory. Some products call a 10-stop filter (a factor of 1,024) "ND1000".

What happens to the shutter speed with an ND filter?

It gets longer by the factor. With ND8 on a 1/60 s exposure, the same brightness needs 1/60 × 8 = about 0.133 s (about 1/7.5 s).