Logarithmic Interpolation Calculator

Calculate linear, log-x, log-y, and log-log interpolation between two known points. Solve forward for Y or reverse the calculation to solve for X.

Solve for

Interpolation mode

Use when both X and Y are plotted on logarithmic axes.

X₁

Y₁

X₂

Y₂

Target X

Number format

Decimal places

Which interpolation mode should I use?

Mode	Use when
Linear	Both X and Y are represented on linear scales.
Log-X	X is logarithmic and Y is linear.
Log-Y	X is linear and Y is logarithmic.
Log-Log	Both X and Y are represented on logarithmic scales.

Worked engineering example

Suppose a PSD curve contains the points 1500 Hz, 2.44 g²/Hz and 2000 Hz, 1.00 g²/Hz. To estimate the PSD at 1731.5 Hz on a log-log segment, enter those two points, choose Log-Log, and solve for Y.

Can I solve in reverse?

Yes. Choose X from Y, enter the same two known points, then provide a target Y value. The calculator rearranges the interpolation relationship and solves for X.

Does the logarithm base matter?

No, as long as the same logarithm base is used consistently throughout the calculation. This calculator uses base-10 logarithms because they are familiar in engineering work and map directly to Excel's LOG10() function.

Can I use scientific notation?

Yes. Choose Scientific to force scientific notation, Standard to force ordinary decimal notation, or Auto to switch automatically for very large or very small values. The calculation itself always uses the full numeric precision of your inputs.