Calculate Subtotal from Total and Percentage
By Michael Henderson •
If I know my tax rate, and I know the total, I'm looking for the subtotal.
So if (subtotal + (subtotal * taxrate)) = total, and I have total and taxrate, how do I get subtotal?
$\endgroup$1 Answer
$\begingroup$Like many mathematicians, I prefer shorter variable symbols. So let $S$ be the subtotal, and let $T$ be total.
Let the tax rate be $r$. If for example the rate is $23\%$, then use $r=0.23$.
We have by your equation $$S+Sr=T.$$ this can be rewritten as $$S(1+r)=T.$$ Divide both sides by $1+r$. We get $$S=\frac{T}{1+r}.$$
Example: Let the tax rate be $23\%$, and let the total be $10000$. then the subtotal is $\dfrac{10000}{1.23}$. This is about $8130.08$.
$\endgroup$
