Convert mixed measurements in one step
Decimal feet = whole feet + inches ÷ 12. For 10 ft 6 in, the result is 10 + 6 ÷ 12 = 10.5 ft. For 8 ft 3 in, it is 8.25 ft. A half-inch is 0.5 inches, so 7 ft ½ in is approximately 7.0417 ft.
Keep a written measurement next to its unit. A note that says only “4” could mean 4 inches of thickness or 4 feet of depth. That difference changes the calculated volume twelvefold. Do the unit conversion before multiplication.
Work through a mixed-unit slab
Consider a rectangular form measuring 10 ft 6 in by 8 ft 3 in, with a specified 4-inch concrete thickness. Its area is 10.5 × 8.25 = 86.625 square feet. Thickness in feet is 4 ÷ 12. Volume is therefore 28.875 cubic feet, or approximately 1.0694 cubic yards.
A selected 10% allowance would increase the target to approximately 1.1764 cubic yards. Purchase rounding comes after this step. The concrete calculator handles the conversion while showing the resulting volume.
Measure sections and depth separately
Sketch an irregular patio as rectangles that do not overlap. Label each part with length, width and concrete depth. Measure along more than one position when edges or the prepared base vary. A nominal four-inch slab over a low spot will consume extra concrete there.
For a deliberately sloping, uniform-width section with a straight-line depth change, the average of its two end depths gives a volume estimate. Random hollows need additional measurements; averaging two convenient spots can miss them. Keep gravel depth out of concrete thickness. Review base quantity separately, and ask the installer to confirm the final measured geometry before ordering. Photograph the labeled sketch for your supplier conversation.
Sources & further reading
Examples illustrate arithmetic. Use project specifications and current local supplier quotes for a purchase.