How do ND filter stops relate to the ND number?

The number of stops equals the base-2 logarithm of the ND number. ND2 is 1 stop, ND4 is 2 stops, ND8 is 3 stops, and so on doubling each time, so ND1000 is about 10 stops and ND100000 is roughly 16.6 stops. Each stop doubles the required exposure time.

How is the new shutter speed calculated?

New time equals the base shutter speed multiplied by 2 raised to the total number of stops. A 1/250 second base with a 10-stop ND filter becomes (1/250) × 1024 ≈ 4 seconds. This tool does the multiplication for you and formats the result.

Can I stack two ND filters?

Yes. Stacking adds the stop values together, so an ND1000 at 10 stops combined with an ND8 at 3 stops gives 13 stops of total reduction. This calculator lets you select two filters and sums their stops automatically, though stacking can introduce vignetting and colour casts.

What ND filter do I need for silky water or cloud blur?

Smooth water usually needs exposures of several seconds, often a 6 to 10 stop filter in daylight, while streaking clouds may need 30 seconds or more, calling for a 10 stop ND or stronger. Enter your metered base speed and try different filters to land in the second-to-minute range.

How do I shoot exposures longer than 30 seconds?

Most cameras max out at a 30-second timed shutter, so for longer exposures switch to Bulb mode and time the shutter with a remote release or app. The calculator shows the exact target time in minutes so you know how long to hold the shutter open.

ND Filter Exposure Calculator

Neutral density filters cut the light entering the lens so you can use long shutter speeds in bright conditions, smoothing water, blurring clouds, and erasing crowds. This calculator converts your base exposure into the long-exposure time you need once the filter is fitted.

How it works

An ND filter blocks light by a fixed number of stops, and each stop doubles the exposure time:

New shutter time = base shutter time × 2^(total stops)

ND filters are usually marketed by an “ND number” where the number of stops is the base-2 logarithm of that number:

stops = log₂(ND number)

So ND2 is 1 stop, ND8 is 3 stops, ND1000 is about 10 stops, and ND100000 is roughly 16.6 stops. When you stack two filters, the stops simply add together.

Worked example

A scene meters at 1/250 second without a filter. Fit a 10-stop ND1000:

Factor = 2¹⁰ = 1024
New time = (1/250) × 1024 ≈ 4.1 seconds

Stack an extra 3-stop ND8 on top and you reach 13 stops, factor 8192, giving about 33 seconds — now you need Bulb mode.

Tips and notes

Compose, focus, and meter before fitting a strong ND, because 10-stop filters make the viewfinder nearly black.
Stacking filters adds stops but can also add vignetting and a colour cast that you may need to correct in post.
For any result longer than 30 seconds, switch to Bulb mode and time the shutter with a release.
All maths runs locally in your browser; nothing about your shoot is uploaded.