Cinder Block Calculator

Nominally 16 x 8 x 8 inches. Put a tape on one and you will measure 15 5/8 x 7 5/8 x 7 5/8 instead. The missing 3/8 inch on each face is the mortar joint, which means the nominal size already accounts for the mortar. Use 16x8 here, not the measured size.

About 3 bags of 70 lb pre-mix per 100 blocks on a standard 8-inch wall. Drop to roughly 2 bags per 100 for 4-inch block, and up to about 4 for 12-inch. Set the field above to match your wall thickness, since the difference across those three is nearly double.

No, and the difference matters. Retaining wall systems use purpose-built segmental block, often around 18 in wide with a separate cap unit, engineered to hold back earth. CMU cinder block is for foundation walls, garden walls, and general masonry, and takes no cap course. If you are holding back a slope, use the Retaining Wall Calculator instead.

About 7% on hollow CMU. The cores leave thin webs at the corners and edges, so blocks chip in handling and crack when cut far more readily than solid units do. That is the loss the 7% is covering.