How it is worked out
Volume is length × width × thickness, all in metres. Thickness is entered in millimetres and divided by 1,000, so a 100 mm slab is 0.1 m thick.
The bag count is the volume multiplied by the number of bags that make one cubic metre, rounded up to a whole bag.
This page works out quantity only. Slab thickness, reinforcing and whether you need a permit depend on what the slab carries and where it is, so check those with your council or an engineer.
Worked example
The numbers this page opens with: length 3 m, width 3 m, thickness 100 mm, extra for uneven ground and spills 5 %, 20 kg bags per cubic metre 108 bags.
The answer: slab area 9.00 m²; volume, exact 0.90 m³; volume with 5% extra 0.95 m³; 20 kg bags 103 bags; weight of those bags 2,060 kg.
Questions
Should I use bags or order a truck?
Look at the weight line in the result. Every bag has to be carried, opened and mixed by hand, so the total weight tells you quickly whether bags are realistic for your job.
Why add extra?
Ground is rarely dead flat and formwork bows a little, so the real hole is usually bigger than the drawing. Running short part way through a pour is far worse than having a barrow left over.