How this calculation works
The requested x determines a fraction of the reference x span. Apply that same fraction to the y span. Outside the reference range, the same arithmetic becomes extrapolation and is labeled accordingly.
y = y₀ + (x − x₀) × (y₁ − y₀)/(x₁ − x₀)A worked example
Between (0, 0) and (10, 100), x = 2.5 gives y = 25.
Good to know
An estimate under the stated inputs and model. Displayed decimals are rounded; verify assumptions before relying on the result.
Halfway in x means halfway in y only under the line model
Between (0, 0) and (10, 100), x = 2.5 sits one quarter of the way across the x interval. Applying the same fraction to the y interval gives 25. The arithmetic assumes a straight relationship between the references.
A real curve can pass through the same endpoints while giving another value in between. Two reference points do not prove that the underlying process is linear.
Outside the interval, the calculation is extrapolation
At x = 15 in that example, the line gives 150. The tool labels it extrapolation because the requested value lies beyond the two references. The formula remains defined, but the assumption is being extended into an unobserved range.
The two reference x values must be different. Otherwise there is no unique y-as-a-function-of-x line to interpolate. Units should be consistent across both x references and both y references.
A few questions, answered
Can the references be entered in reverse order?
Yes. Reversing both complete points leaves the line unchanged.
Does it fit a curve to more points?
No. It uses exactly two reference points and a straight-line model.