Linear Interpolation Calculator

Understanding your result

The position expresses how far the target lies from the first point toward the second. A value of 50% is halfway; negative values or values above 100% are outside that interval.

For a calculation combining several values and weights, use the Weighted Average Calculator .

The formula

First compute the target’s fraction of the horizontal interval. Apply the same fraction to the change in y, then add the first y value. Both endpoints must use compatible units, and the two x values must differ.

How to use this calculator

1. Enter the two known x and y pairs in consistent units.
2. Enter the target x, keeping each y paired with its original x.
3. Read the estimate and check whether it is inside or outside the known interval.

When a straight line is a reasonable approximation

Interpolation fills a gap under an explicit model; it does not reveal an unknown physical law. A narrow interval in a smooth table may be well approximated by a line, while a sharp bend or threshold may not. If the underlying table specifies logarithmic interpolation or another method, this linear calculation is not interchangeable with it.

Extrapolation deserves separate interpretation

Outside the endpoints, the same algebra extends the line indefinitely. The displayed label makes that distinction visible, but it does not measure the uncertainty. Reversing the order of the two pairs produces the same estimate. This tool handles one segment; it does not select surrounding points automatically from a larger dataset or fit a regression to noisy observations.

Assumptions & limitations

What this calculation assumes

• The change in y is linear over the chosen x interval.

What to keep in mind

• No uncertainty bounds, polynomial interpolation, logarithmic scale, or regression fitting.

Common questions