01
How to use this calculator
- Enter two known coordinate pairs.
- Choose whether you want to solve for x or y.
- Enter your target value and calculate the point on the connecting line.
02
A little understanding goes a long way.
Linear interpolation assumes a straight-line relationship between two known observations. A target outside the known interval is extrapolation, which may be less reliable.
THE FORMULA
y = y₁ + (x − x₁)(y₂ − y₁) ÷ (x₂ − x₁)
TRY IT ON PAPER
A worked example
Between (10, 20) and (30, 60), the value at x = 15 is y = 30. The point lies one quarter of the way between the two known x-values.
03
A few common questions
What happens outside the interval?
The calculator still evaluates the line, but labels the result as extrapolation. The linear relationship may not hold beyond the known points.
Can I solve for x instead?
Yes. Select Find x and enter a target y. The inverse calculation requires the two known y-values to be different.
Why must the known x-values differ?
Two points with the same x-value do not define a single-valued linear function y(x). Enter points with different x-coordinates.