About the Focal Length Calculator
Focal length from object and image distances, from lens power, or from the lensmaker's equation.
Also known as: focal length formula, lensmaker equation, diopter to focal length.
How to use this calculator
- Enter calculate from and the other fields.
- The result updates instantly as you type — press Calculate (or Enter) at any time.
- Review the breakdown shown under the main result.
- Use Copy result or Share to save the answer or send a link with your inputs.
Formula
1/f = 1/u + 1/v · P = 1/f(m) · Lensmaker: 1/f = (n−1)(1/R₁ − 1/R₂)
Worked example
Using the values pre-filled in the calculator above:
- Calculate from: Object and image distance
- Object distance (cm) = 30
- Image distance (cm) = 15
the calculator returns:
- Focal length: 10 cm
- In metres: 0.1
- Lens power: 10 D
- Lens type: Converging (convex)