When ordering materials it would be better to order a little extra than too little as you could be left with a structural weakness where two different batches of concrete join.
Firstly, calculate the volume of the concrete you will need which may involve compiling a simple plan. Using graph paper, choose a scale such as 1:10 (1 cm = 10 cm) or 1:20. Use a pencil first, then ink after all the editing are done.
Then calculate the area and multiply this by the depth but be careful not to misplace a decimal point though.
A building contractor may use a half a dozen different mixes but a smaller residential project only two may be needed.
The first, or coarse mix, will be used for foundations, driveways and floors and may be referred to as a 1:2,5:4 mix, in other words, 1 part cement to 2.5 parts sand to 4 parts aggregate which should be of a course variety.
The second, or fine mix, is used for paving stones, footpaths and other thinner concrete sections. This is made from a 1:2:3 mix of 1 part cement, 2 parts sand and 3 parts fine aggregate.
You can dispense all this by ordering ready-mix concrete especially if you do not need much concrete and additional costs of hiring equipment as labour could be a reality.