# Thread: Formula Needed

1. ## Formula Needed

Hi All

I need a formula for a hobby of mine. I use clay to make things. As it dries- it also shrinks. Depending on which type of clay I use- it can shrink anywhere between 10%-28%.

My question is this (as an example)- assuming I want to wind up with a dried ,finished product with diameter of 25 cms and the clay I'm using shrinks by 12%- What would I need to start off with before I start the drying process? I know most people will multiply 25cms times 12% - then add it to the 25cms for the answer. But 3cms + 25cms= 28cms. 28cms x 12%=3.36cms 28cms minus 3.36cms=24.64cms.
Close - but not close enough.

Anyone have a formula that will work more precisely?

Thanks

Robert

2. Originally Posted by Goldfinger
Hi All

I need a formula for a hobby of mine. I use clay to make things. As it dries- it also shrinks. Depending on which type of clay I use- it can shrink anywhere between 10%-28%.

My question is this (as an example)- assuming I want to wind up with a dried ,finished product with diameter of 25 cms and the clay I'm using shrinks by 12%- What would I need to start off with before I start the drying process? I know most people will multiply 25cms times 12% - then add it to the 25cms for the answer. But 3cms + 25cms= 28cms. 28cms x 12%=3.36cms 28cms minus 3.36cms=24.64cms.
Close - but not close enough.

Anyone have a formula that will work more precisely?

Thanks

Robert
hmmm.

$25=(1-.12)a$

Therefore: $25=0.88a$

Divide: $\frac{25}{0.88}=a$

Finally: $28.4090909=a$

Therefore the formula is: $\boxed{\frac{\text{the amount wanted}}{1-\text{the percent loss}}=\text{the amount you should start with}}$