THE SHORT ANSWER
Calculate a rectangular shed base from measured length, width and the thickness specified for your project. The volume calculation estimates concrete quantity; the required slab design, support and thickness need separate confirmation.
Record the specified geometry
Write down the shed footprint and the actual slab dimensions in ft. They may differ where a designer specifies an apron or edge detail. Measure the slab rectangle, not the roof projection. Record the drawing revision and the source of the thickness input so a quantity change can be traced to a scope change.
Thickness is an input to this calculation, not a recommendation. Shed loads, ground conditions, edge details and other design decisions cannot be determined from footprint alone. Confirm the required specification before ordering. Do not copy the illustrative thickness below simply because the example has a similar shed size.
Calculate concrete volume
Volume in cubic feet = length in feet × width in feet × thickness in inches ÷ 12. Divide cubic feet by 27 for cubic yards. Multiply that volume by your chosen ordering allowance. The calculator shows both geometric volume and quantity with allowance so they remain distinguishable.
Illustrative geometry: 24 ft × 16 ft × 4 inches gives 128 cubic feet, or about 4.74 cubic yards. A chosen 10% quantity allowance gives about 5.21 cubic yards. These dimensions are arithmetic inputs, not an approved structural design or a supplier order.
Account for thickened edges and delivery
A simple rectangle does not include a thickened perimeter, footing, ramp or isolated pad. Measure each additional shape and add only the volume outside the slab already counted. For a thicker edge, use its extra depth rather than counting the full depth twice. Keep each component visible in the takeoff.
Ask the concrete supplier about order increments, minimum charges, access and delivery arrangements. Discuss the allowance with the person responsible for the pour rather than assuming one percentage fits every job. If using bagged material, divide the required volume by the yield stated on the chosen product, in matching units, and round up to whole bags.
Build the shed base quote around the quantity
The concrete volume is one line of a larger itemized estimate. Separate excavation, disposal, compacted base, formwork, specified reinforcement, placing, finishing, delivery and any equipment. Price concrete with a current supplier quote in USD; do not treat the example PDF as a local cost survey.
The United States sample shows a fictional completed quote with stated scope and exclusions. Compare its assumptions with your own drawings before reusing a line. Confirm who provides access, who handles spoil and what happens if ground conditions differ. Keep taxes, overhead and profit outside the quantity calculator so an order volume is never mistaken for a selling price.
Common questions
What thickness should a concrete shed base be?
Use the thickness specified for your shed, loads and ground conditions by the appropriate designer or qualified contractor. This calculator accepts that input and does not choose or certify it.
Does the calculator include the sub-base?
No. Concrete slab volume and compacted sub-base quantity are different takeoffs. Measure and price the specified base layer separately.
FROM VERIFIED INPUTS TO AN ESTIMATE
Create your own estimate from the reviewed scope
Use Estimates on iPhone or Android to draft a scope with AI assistance or enter your own items. Review quantities, rates and exclusions before exporting a PDF. The completed sample is free to inspect; some app features may require an in-app purchase.
