Best to use some brewing software:
Basically each pound of malt has an extract potential. Say, 2-row has about 36 points per pound, per gallon. So ONE pound in ONE gallon at 100% eff. would be an OG of 1.036
If you had 70% eff. in that same gallon, youd have an OG of 1.025
SO, if you have 10 pounds of 2-row, that is 360 potential points of gravity in ONE gallon. If you are making 5 gallons you divide that by 5... that is 1.072 if you have 100% eff. Say your OG is actually 1.050 in 5 gallons with 10 pounds of 2 row... that is 50/72 which is 70% eff.
Brewing software helps, but it can be done with a little math by hand.
It is your ACTUAL OG divided by the POTENTIAL OG of the brew. Each malt has a specific # of points/lb. Most are between 34 and 36 points, meaning 1.034 to 1.036 per pound of malt, in one gallon, at 100% eff.
You will get about 70% of the total as a good starting point.