Linear interpolation between two known points — plus extrapolation warnings and the inverse solve (find x for a given y).
If the known value lies outside the interval between the two points, the result is an extrapolation and is flagged as less reliable.
interpolated result
How to use this tool
y for a given x, or x for a given y (inverse).
Both points define the straight line used for the estimate.
The x (or y) you want to evaluate.
Values inside the interval are interpolated; values outside are flagged as extrapolations.
Formula & worked example
Points (0, 0) and (10, 100), x = 4: y = 0 + 4 x 100/10 = 40. At x = 12 the same line gives 120, but that is an extrapolation beyond the known data and is flagged.
Limitations — read this before you rely on it
Linear interpolation assumes a straight line, which real data rarely follows exactly; the further outside the interval, the less trustworthy the result. It also cannot be used when x1 equals x2 (a vertical line).
Frequently asked questions
Estimating a value between two known points by assuming a straight line joins them.
Estimating outside the range of known data by extending the same line — useful, but riskier than interpolation.
When the underlying relationship really is linear between the points; otherwise it is an approximation.
Yes, with splines or polynomial interpolation, but this calculator uses the standard two-point linear method.
It runs the same line equation backwards: given y, it finds the x on the line with that y-value.