# How do I extrapolate data from a NON-LINEAR (logarithmic) standard curve in Excel?

I have made a standard curve. The X-axis is logarithmic. The y-axis is linear. I have added a logarithmic trendline (y = -1.546ln(x) + 39.254; R² = 0.9906). How can I re-arrange the equation to calculate an unknown X-axis value?

• *from a known y-value – Jessica Oliver-Bell Apr 30 '13 at 13:25
• just copy the equation you got into excel cell and place in it instead of X - put the cell name that contains the X value you want – user20965 Jul 10 '16 at 14:14

## 1 Answer

$$y = -1.546 \ln(x) + 39.254$$ $$\frac{39.254 - y}{1.546} = \ln(x)$$ $$\exp\left(\frac{39.254 - y}{1.546}\right) = x$$

In excel this is =EXP((39.254 - Y)/1.546) where Y is the cell address containing the Y value.