The block count is the easy part
Every concrete block calculator agrees on 1.125 blocks per square foot, and every one of them is right, because it is not an estimate at all. ASTM C90 sizes a hollow unit at 7-5/8 by 15-5/8 inches precisely so that the block plus one 3/8 inch mortar joint lands on a round 8 by 16 inches of wall. Divide one square foot by 0.8889 and you get 1.125. There is no judgement in it and no supplier variation to worry about.
What separates a useful takeoff from a useless one is everything after that number. How many bags of mortar. How much grout, if the wall is reinforced. Whether the figure you are quoting is a manufacturer's published yield or something a calculator author reverse-engineered and rounded. That is where most of the pages competing for this search quietly stop citing anything, and it is the part this tool takes seriously.
Read the words around the mortar figure
QUIKRETE's data sheet for Mortar Mix says an 80 lb bag "will lay up to 37 standard bricks or 13 standard (8 in by 8 in by 16 in) blocks." Nearly every site repeating that 13 drops the words up to, and the two versions mean different things. Thirteen is what a bag achieves with a tight, consistent joint and no waste on the board — a ceiling, not a mean. Buttering generously, working in heat that stiffens the mud, or throwing away what goes off before it is used all push the real number down.
There is a second thing that figure quietly assumes: an 8 inch wide unit. Bed joints run the full width of the block, so a 12 inch unit takes half again as much mortar per course as an 8 inch one, and the published 13 does not know that. This calculator applies the yield as published and tells you what it applies to, rather than inventing a width correction that QUIKRETE has never printed.
Where the per-core grout volume comes from
Grout is quoted by the bag, but a wall is grouted by the core, so something has to connect the two. QUIKRETE publishes both halves: an 80 lb bag of Core-Fill Grout Coarse yields 0.65 cubic feet, and it fills the cores of approximately three standard blocks. Three blocks at two cores each is six cores, so a core holds about 0.108 cubic feet. That is the number driving the grout output, and the working shows the division rather than hiding it.
Because cores sit on 8 inch centres, reinforcement spacing maps cleanly onto a count of filled cores: 16 inches on centre is one core per block, 32 inches is one core every second block, 48 inches is one in three. Solid grouting fills all of them. That relationship is why the spacing dropdown offers the multiples it does — a two-core unit physically cannot put steel at 20 inches on centre.
What this will not tell you
It will not tell you whether your wall needs reinforcement, what size bar, at what spacing, or whether it must be solidly grouted. Those are engineering and code questions, they vary by seismic and wind region, and a calculator that answered them would be doing something genuinely dangerous. Bring the spacing from your drawings; the tool will turn it into a quantity. Once you have the bar schedule, the rebar calculator handles lineal feet, lap splices and weight.
It also will not give you a core-fill quantity for 6, 10 or 12 inch units, because the published coverage figure names the 8 inch block and nothing else. Scaling it by width would produce a confident-looking number with no source behind it, sitting on the same page as figures that do have one — which would make the sourced figures worth less. Block count and mortar still work for every width. For the grout, ask your unit manufacturer for the net cross-sectional area.
For a mortared wall in clay brick rather than block, the brick calculator uses the Brick Industry Association quantity tables instead, and for the footing under all of this the concrete bag calculator covers bagged mixes.