Roof.... As Barry said, and with only addressing the mathmatics of your question, and ignoring any advice with regard to a business perspective,Maybe thats my problem, I can't explain it so I cannot calculate it.
One does'nt necessarily know the selling price, you know the costs and your margin is 25%, i.e. you want to make a 25% margin not 25% mark up.
A. 75 x .25 = 18.87 + 75 = 93.75 (.25 represents 25% mark up)
B. 75 + 25% = 100.00
I guess this is one reason why tradesmen struggle with the complicated numbers. Doesn't seem that complicated now.
Aside form making additional $6.25 per $100...
How do you actually get the margin number of 25% or whatever it may be? Speculation guesstimate?
I believe your question was:
If I know my cost of a project, and I know the margin that I want to make, how do I calculate the selling price.
The equation is as Barry indicated:
Selling price = cost/ (1- margin)
Let's say you figure your costs to do a job is $1500. And as you said you want to make a 25% margin.
So your Selling price= $1500 (your cost)/ (divided by) 1-.25 (your margin)
Remember when you say your margin is 25%, that is mathmateically .25
So Selling Price = 1500/.75=$2000
NOW... Pleae do not get lost/mixed-up in the math as it concern business principals... it is important to understand the math and the terminolgy, but only in so far as it helps understand underlying principals... which others are addressing above.
Relative to what Barry is speaking about above, apparently his 7% cost of delivery,... that is meerly to say that you may not have figured your cost correctly... and your cost in the above calculation should have been 1500 + (.07*1500)= $1605 (which would have given you a $2140 selling price.
But again, that does not address business principals, it is just about the math and terminology we use in business.
More about that later.....