Why estimate materials first?
Buying too little means a second trip and, for products like tile or paint, a risk of mismatched batches. Buying far too much wastes money. A quick estimate with a sensible waste allowance gets you close, and the explanation under each calculator shows exactly how the number was reached so you can sanity-check it.
How accurate are the results?
The math is exact for the dimensions you enter. The real-world uncertainty comes from the inputs: uneven ground, rough surfaces, cutting waste and differences between brands. Each calculator lets you set a waste percentage and, where it matters, the coverage printed on the product label. Treat every result as a planning estimate and confirm with the manufacturer's label and your local building code.