The core rule
Combine volume first. Round packages last.
Each slab, circular pad, footing, column, wall, or post hole should be calculated from its own measured geometry. BuildNumbers converts each part to a common physical volume and adds those unrounded values before applying the project allowance.
Vproject = V1 + V2 + … + VnOrder volumeVorder = Vproject × (1 + allowance ÷ 100)Complete bagsbags = ceil(Vorder ÷ bag yield)This avoids a common mistake: rounding each small part to complete bags and then adding those rounded package counts. If the material will be purchased as one combined order, that method can repeatedly add rounding overhead.
1. Break the job into measurable parts
Give each physical part a clear label such as “patio slab”, “round equipment pad”, “west footing run”, or “gate post holes”. Choose the geometry that actually matches the part instead of forcing every item into a rectangular volume.
Repeated identical parts can use a quantity multiplier. If dimensions differ, keep them as separate parts so the calculation record remains auditable.
2. Use the units that match the source measurement
A project can mix Imperial and Metric inputs. The important point is not the display unit; it is that every part represents the same physical quantity after conversion. BuildNumbers uses exact shared unit constants and converts each result before aggregation.
When you switch a part between Metric and Imperial in the project builder, the entered geometry is converted rather than reinterpreted as the same number in a different unit.
3. Keep geometry-specific deductions inside the part
Wall openings and post displacement affect only the part where they belong. A wall should subtract measured full-depth openings before its volume enters the project total. A post hole can subtract the measured post displacement before aggregation.
These deductions should never be applied as a project-wide percentage because they describe real geometry, not uncertainty.
4. Apply one explicit project allowance
After all net part volumes are combined, apply the selected project allowance once. BuildNumbers accepts an explicit 0–50% allowance and does not silently choose one for you.
The appropriate allowance depends on the project, placement method, site conditions, supplier rules, and the consequences of running short. Treat it as a planning input that you must choose and verify.
5. Round the purchase quantity once
For bagged concrete, choose the actual package size and verify the product yield. BuildNumbers supports the documented 40, 60, and 80 lb yield assumptions used by its concrete tools, then rounds the final combined bag count upward once.
Ready-mix orders may have supplier minimums, increments, delivery rules, or practical overage requirements that are not represented by whole-bag rounding. Confirm those rules with the supplier before ordering.
The calculator does not choose slab thickness, footing dimensions, column size, wall thickness, reinforcement, concrete strength, bearing area, frost depth, joints, drainage, formwork, excavation, or any structural/code requirement. Enter geometry from the project design or another qualified source.
A practical audit before ordering
- Confirm every part is represented once and only once.
- Check measured dimensions and repeated-part quantities against the plans or site.
- Verify wall openings and post displacement were entered only where applicable.
- Confirm the project allowance is intentional.
- Check bag yield or ready-mix supplier rules against the product you will actually buy.
- Review the part breakdown so one geometry cannot hide an input mistake inside the grand total.
Primary references
Unit conversions follow the NIST SI conversion guidance. Bag-yield assumptions used by the BuildNumbers concrete family are checked against manufacturer-published concrete mix information.