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Materials are bought in fixed units — tins, boxes, packs — so the useful answer to a DIY question is rarely "how many litres" or "how many square metres". It is how many tins, and how many boxes, with enough spare to finish the job.
These calculators work in those terms. The paint calculator deducts doors and windows rather than pretending walls are blank, multiplies by the number of coats, and rounds up to whole tins. The tile calculator adds a wastage allowance for cuts and breakages and converts the total into boxes.
Two rules of thumb worth knowing before you buy: two coats is the realistic minimum for any colour change, and ten per cent wastage is standard for straight tiling, rising to twenty for diagonal layouts or awkward rooms. Buy from a single batch where you can — paint and tile shades both vary between production runs, which is why a repair a year later so rarely matches.