Meter the scene without the filter, enter that shutter speed and your ND filter strength, and get the long exposure time to use with it. Works with stops, ND numbers (ND8, ND1000) or optical density (0.9, 3.0), and shows the time in mm:ss for your timer.
Last updated:
Meter without the filter; e.g. 1/125, 1/30, 0.5 or 2
stops
e.g. 2 for a polarizer, or another ND.
Enter your values to see the result.
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How to use the ND Filter Calculator
Meter the scene without the filter and note the shutter speed at your chosen aperture and ISO.
Enter that shutter speed, for example 1/125.
Enter the filter strength and choose whether it is in stops, ND number or density.
Add a stacked filter if you use two filters together.
Set the new shutter speed, or use Bulb mode with a remote and the mm:ss timer value for long exposures.
ND filter formula
New shutter time = base time × 2stops. Stops from an ND number (filter factor F): stops = log₂(F). Stops from optical density D: stops = D ÷ log₁₀(2) ≈ D ÷ 0.301.
Example: base 1/60 s with a 6-stop filter → 1/60 × 64 = 1.07 s. Adding a 2-stop polarizer gives 8 stops: 1/60 × 256 = 4.3 s.
ND filter chart
Stops
ND number (factor)
Optical density
Light passed
1/125 s becomes
1/30 s becomes
1
ND2
0.3
50%
1/60 s
1/15 s
2
ND4
0.6
25%
1/30 s
1/8 s
3
ND8
0.9
12.5%
1/15 s
1/4 s
4
ND16
1.2
6.25%
1/8 s
0.5 s
6
ND64
1.8
1.56%
0.5 s
2.1 s
8
ND256
2.4
0.39%
2 s
8.5 s
10
ND1000
3.0
0.1%
8 s
33 s
13
ND8000
3.9
0.0125%
1 min 6 s
4 min 33 s
15
ND32000
4.5
0.003%
4 min 22 s
18 min 12 s
Filter factors are rounded on the box: ND1000 is really 29.97 and a "10-stop" filter is 210 = 1024×. The difference is less than 3% of the time.
Frequently asked questions
How do I calculate shutter speed with an ND filter?
Multiply the base shutter speed by 2 raised to the number of stops: new time = base × 2^stops. With a 10-stop filter, 1/125 s becomes 1/125 × 1024 ≈ 8.2 s.
What do ND numbers like ND8 or ND1000 mean?
The number is the filter factor: how many times less light passes. ND8 passes 1/8 of the light (3 stops), ND64 passes 1/64 (6 stops) and ND1000 passes 1/1000 (about 10 stops).
What is optical density (ND 0.9, ND 3.0)?
Optical density is the base-10 logarithm of the filter factor. Each 0.3 of density is about one stop: ND 0.9 = 3 stops, ND 1.8 = 6 stops and ND 3.0 = 10 stops.
Which ND filter should I buy for long exposures?
A 6-stop filter turns daylight shutter speeds into a fraction of a second to a few seconds, good for silky water. A 10-stop filter gives 15 to 60 seconds in daylight for smooth seas and streaking clouds. 15-stop filters allow minutes-long exposures in bright sun.
What if the time is longer than 30 seconds?
Most cameras only offer timed shutter speeds up to 30 s. Use Bulb mode with a remote release or intervalometer and time the exposure with the mm:ss value shown.
Why is my long exposure darker or color-shifted?
Strong ND filters can leak light through the viewfinder (cover it) and many add a slight color cast that you can fix with white balance. Digital sensors do not suffer reciprocity failure, but film does, so add time with film according to its datasheet.
Are all the tools on FreeToolCabin tested for quality and accuracy?
Yes! Every tool on our site undergoes automated Q/A testing in real browser environments to ensure high precision, correct output, and zero errors. If you happen to spot a bug or have a suggestion, please contact us at [email protected] and our team will review it right away.