Sensor Size to Crop Factor Calculator

Crop factor describes the size ratio between a camera sensor and the 35mm full-frame standard (36mm × 24mm). This page runs the formula in the direction the others do not: give it a sensor’s width and height in millimetres and it tells you the crop factor, rather than making you already know it. That is the useful direction for phones, drones, action cameras, cinema formats, and anything whose spec sheet lists dimensions but no multiplier.

If you already know your crop factor and just want to see what a lens does on it, use the lens crop factor calculator, which puts one lens across every format at once. To convert a focal length in either direction, focal length equivalent does that.

Crop Factor formula: Crop Factor = Full-Frame Diagonal / Sensor Diagonal

Full-Frame Diagonal = √(36² + 24²) = √(1296 + 576) = √1872 = 43.27 mm

So a Micro Four Thirds sensor at 17.3 × 13 mm has a diagonal of √(17.3² + 13²) = 21.64 mm, and 43.27 ÷ 21.64 = 2.00×. That is where the number on every MFT spec sheet comes from.

Equivalent Focal Length: Equivalent FL = Actual Focal Length × Crop Factor

Equivalent Aperture (for depth of field only, never for exposure): Equivalent f-number = Actual f-number × Crop Factor

What each variable means:

• Crop Factor, the multiplier that converts an actual focal length to its full-frame equivalent field of view. The calculator derives it from your dimensions, so it returns 1.53× for APS-C rather than the 1.5 everyone quotes, and 1.61× for Canon APS-C rather than 1.6. Those rounded numbers are marketing shorthand; the diagonals are the real thing.
• Equivalent Focal Length, the full-frame focal length that produces the same angle of view; a 50mm on Canon APS-C looks like an 81mm on full frame
• Equivalent Aperture, the f-number that produces equivalent background blur at matched framing; f/2.8 on APS-C gives the same depth of field as f/4.3 on full frame. It does not convert exposure. An f/2.8 lens meters as f/2.8 on every camera ever made, and your shutter speed does not change when you switch bodies.

Worked example: Fujifilm X-T5, whose sensor measures 23.6 × 15.6 mm. Lens: 23mm f/1.4.

Diagonal = √(23.6² + 15.6²) = 28.29 mm, so crop factor = 43.27 ÷ 28.29 = 1.53× Equivalent FL = 23 × 1.53 = 35mm (the classic street-photography look on full frame, which is exactly why Fujifilm chose 23mm) Equivalent aperture = f/1.4 × 1.53 = f/2.1 (for depth of field comparison)

So the lens meters as a genuine f/1.4 and you get f/1.4 shutter speeds. What you do not get is f/1.4 background blur at full-frame framing: the blur matches a 35mm f/2.1 lens on full frame, which is about a stop and a bit less separation than a true 35mm f/1.4 would give you. Still shallow, just not as shallow as the number on the barrel suggests.

Sensor dimensions worth typing in, with the factor each one produces:

Phone sensors are the ones worth checking rather than assuming, and the reason is in that table: a 1/2.55-inch chip crops two thirds harder than a 1/1.3-inch one, which is the difference between a 24mm equivalent and a 40mm one behind the same physical lens. Manufacturers quote the equivalent, not the actual, so you cannot work backwards without the dimensions.

Fractional-inch sensor names do not mean what they look like. A “1-inch” sensor has a 15.86 mm diagonal, which is well under an inch. The naming is a leftover from 1950s vidicon tubes, where the number was the outside diameter of the glass envelope rather than the imaging area. Roughly, the imaging diagonal is about two thirds of the quoted inch figure. This is exactly why measuring beats guessing, and why the table above gives millimetres.

Area, and the part crop factor hides. Crop factor is a ratio of diagonals, so sensor area falls with roughly its square. Micro Four Thirds at 2.00× holds 225 mm² against full frame’s 864, which is 26% of the area and just under two stops of total light at matched framing. Not half, which is what a bare “2×” tends to suggest to people.

Why 26% rather than exactly 25%: Micro Four Thirds is a 4:3 shape and full frame is 3:2, and for a given diagonal the squarer rectangle encloses slightly more area. That is also why the format holds up better than the multiplier implies. The calculator prints the real area and the real stop difference for whatever dimensions you give it, rather than assuming everything is 3:2.

When crop factor helps: APS-C and MFT sensors give telephoto reach at lower weight and cost. A 300mm f/5.6 on APS-C equals 450mm full-frame reach, which is valuable for wildlife and sports.