How do I convert 1/125 to seconds?

Divide 1 by 125, which is 0.008 seconds, or 8 milliseconds. The converter does this for any fraction and also reverses it, turning 0.008 seconds back into roughly 1/125.

What is the EV difference between two shutter speeds?

Each doubling of exposure time is one stop more light. The difference in stops is the base-2 logarithm of the ratio of the two times. Going from 1/125 to 1/250 halves the time, a one-stop reduction in light.

Why are shutter speeds shown as fractions?

Most action and handheld speeds are faster than one second, so they are written as fractions of a second. 1/500 means one five-hundredth of a second. Speeds of one second or longer are shown as whole seconds or with a quote mark.

What is the nearest standard shutter speed?

Cameras step shutter speed in thirds of a stop along a fixed ladder. The tool snaps your value to the closest marking on that ladder, which is handy when a meter or formula gives an in-between time you must round to a real setting.

How does this help with ND filters?

An ND filter adds a known number of stops. Multiply your metered shutter time by 2 to the power of the filter strength to get the long-exposure time. Use the stop-difference output to check that two times differ by the filter's rating.

Shutter Speed Fraction to Decimal Converter

This converter turns camera shutter speeds between the fraction form on your dial (1/125, 1/4000) and plain decimal seconds, and it tells you how many stops separate any two speeds. It is the small piece of exposure arithmetic that ND-filter, bulb-timing, and flash-sync math all depend on.

How it works

A shutter speed is simply a length of time. The fraction 1/125 means 1 ÷ 125 = 0.008 seconds. The converter parses three input forms:

A fraction such as 1/250 becomes 1 ÷ 250 = 0.004 s.
A decimal such as 0.004 is used directly.
A whole number such as 30 is treated as 30 seconds.

To express any time back as a fraction, it inverts: 1 ÷ 0.004 = 250, so the result is 1/250.

Stops between two speeds

A “stop” is a doubling or halving of light, and shutter time scales light linearly, so the difference in stops between two times t1 and t2 is:

stops = log2(t1 / t2)

Going from 1/125 (0.008 s) to 1/500 (0.002 s) is log2(0.008 / 0.002) = log2(4) = 2 stops less light — exactly what you would gain back by opening two stops of aperture.

Worked example: a 6-stop ND filter

You meter a scene at 1/60 s and want a long exposure with a 6-stop ND filter. Each stop doubles the time, so multiply by 2^6 = 64:

1/60 s × 64 = 64 / 60 ≈ 1.07 s

So your shutter goes from 1/60 to about 1 second. The stop-difference tool confirms log2(1.07 / 0.0167) ≈ 6 stops, matching the filter rating.

Notes

Standard cameras step shutter speed in thirds of a stop along a fixed ladder (... 1/125, 1/160, 1/200, 1/250 ...). The tool snaps your value to the nearest real marking so a calculated in-between time maps to a setting you can actually dial in. Everything runs locally in your browser.