Concrete Allowance and Order Rounding: Keep Them Separate

Start with measured volume, then choose an allowance

The target formula is net volume × (1 + allowance percentage ÷ 100). A measured 4.94 cubic yards with 10% extra has a target of 5.434 cubic yards. An allowance does not fix an omitted footing, incorrect unit or unmeasured low area.

NRMCA's ordering guidance discusses a contingency above plan dimensions, with final quantity planning tied to the actual job. Treat our allowance field as an editable planning choice, not a universal instruction to order the same percentage for every project.

Round only after the required additions

Suppose the measured net quantity is exactly 4.94 yards, the selected allowance is 10% and the supplier sells quarter-yard increments. Round the 5.434-yard target upward to 5.50 yards. The difference from net volume is 0.56 yards, which includes the allowance and final rounding.

Do not round the net number to 5.00 first, add 10%, then round again without recognizing what that does. Similarly, applying 10% twice gives 21% overall, not 20%, because the second increase acts on the first increased quantity.

The concrete calculator shows net and allowance quantities separately. Use the appropriate buying tool for whole bags or a quoted order increment.

Avoid double counting in combined projects

If three sections use the same mixture and share one delivery, add their geometric volumes before applying a common allowance and purchase rounding. If sections need different products or separate deliveries, keep their orders separate.

For blockwork, a purchased breakage allowance should increase the number of spare blocks. It should not automatically enlarge the wall's actual area or the volume of its filled cores. For premix, use the bag calculator to compare target volume with the final bags' total labeled yield.

Record your assumptions and discuss the final quantity with the installer or supplier. Planning for extra material also means planning where any surplus can be handled under the site's agreed arrangements.