MinuteMetric

Estimating Materials for Home Projects

Underestimating materials is the most reliable way to turn a weekend job into a fortnight. A second trip to the merchant costs time; a second batch of paint from a different mixing run can be visibly different on the wall; a slab poured in two sessions has a permanent weak joint through it. These calculators exist to get the quantities right before you start.

The arithmetic is straightforward — areas and volumes — but two things trip people up. First, waste: no project uses 100% of what you buy, because tiles get cut, boards get trimmed and some material is always lost. Second, coverage figures are stated for ideal conditions, and a porous, textured or previously unpainted surface will absorb noticeably more than the tin claims.

Practical Guidance

• Add a waste allowance. Roughly 10% for straightforward rectangular rooms, and 15–20% where there are angles, alcoves, or a diagonal or patterned layout.
• Measure twice, in more than one place. Rooms are rarely square and walls are rarely plumb. Measure each wall rather than assuming opposite walls match.
• Buy in one batch. Paint, tiles and flooring can vary in shade between production runs. Buying the full quantity at once avoids a mismatch you'll notice forever.
• Keep the offcuts. Spare tiles and boards are invaluable for repairs years later, when the product line has been discontinued.
• Check the code. Anything structural, electrical or involving gas is governed by regulations, and often requires a qualified professional.