Linear Interpolation Calculator
Enter two known points and the x value you want y for.
This result is an estimate only. Double-check important figures before relying on them.
How it is calculated
- Slope = (y₂ − y₁) ÷ (x₂ − x₁)
- y = y₁ + (x − x₁) × slope
For the full line equation, use the line equation calculator.
Worked example
Points (1, 10) and (3, 20), at x = 2:
- y = 10 + (2 − 1) × 5 = 15
Why your number may differ
Real data is often curved, so the straight-line estimate can be off, especially when the points are far apart.
Frequently asked questions
What is linear interpolation?
Estimating a value between two known points by assuming they lie on a straight line.
What is the formula?
y = y₁ + (x − x₁) × (y₂ − y₁) ÷ (x₂ − x₁).
When is it unreliable?
When the real relationship is curved, or when you extrapolate far outside your data.