About this calculator
Enter the two known points and the x you need, or switch to find the x for a given y.
How to use it
- Enter the two known points.
- Enter the x (or y) you need.
- Read the interpolated value.
The formula
y = y₁ + (x − x₁) × (y₂ − y₁) ÷ (x₂ − x₁)
- (x₁, y₁), (x₂, y₂)
- the two known points
- x
- the point where y is wanted
Worked example
Points (10, 200) and (20, 260), find y at x = 14
- Slope = 60 ÷ 10 = 6.
- y = 200 + (14 − 10) × 6 = 224.
What the result means
If x lies outside the two points the result is an extrapolation, which is less reliable.
Assumptions
- The quantity changes in a straight line between the points.
Limitations
- For strongly curved data, use closer points or a higher-order method.
Frequently asked questions
What is the interpolation formula?
y = y₁ + (x − x₁)(y₂ − y₁) ÷ (x₂ − x₁).
What is the difference between interpolation and extrapolation?
Interpolation estimates inside the range of known points; extrapolation goes beyond it.
What is double interpolation?
Interpolating in two directions of a table: do it once along each row, then once between the two results.
Last reviewed on 7 October 2026. Found a mistake? Tell us.