Formula
m = (y₂ − y₁) / (x₂ − x₁) y = y₁ + m × (x − x₁)
Example
1,200 in 2023 and 1,500 in 2025. Estimate 2028.
m = 300 / 2 = 150 per year → y = 1200 + 150 × 5 = 1950
Interpolation vs. extrapolation
Both use the same line. Between the points it's interpolation and generally safe. Outside them it's extrapolation, which is only as good as the assumption that the trend continues.
FAQ
How reliable is extrapolation?
Less reliable than interpolation. The further you go from your data, the more the real trend can bend away from a straight line.
Which two points should I use?
Usually the two closest to the value you are estimating, such as the last two measurements when forecasting.