Why Cost Estimation Decides Whether You Profit

On a construction project, materials are often 60% or more of the total cost. At that share, a 10% estimating error on a $50,000 house extension is $3,000 or more gone: either eaten from your margin or demanded from the client mid-project, which is how disputes start. Good estimation isn't about precision to the last bag; it's about a repeatable process that catches everything once.

The Process: Bill of Quantities First, Prices Second

Separate the two jobs. First work out how much of everything you need (quantities), then find out what it costs (rates). Mixing them is how items get forgotten.

  1. Take off quantities: from drawings or site measurements, list every material in standard units: cement in bags, sand and gravel in m³, steel in kg, bricks in numbers, tiles and paint in m².
  2. Build the Bill of Quantities (BOQ): one row per material: description, unit, quantity. Nothing else yet.
  3. Collect rates: price each row from at least three suppliers. Prices move; a quote from six months ago is fiction.
  4. Add wastage: 5–10% on most materials (more below).
  5. Add contingency: 10–15% on the total for price changes, rework and the things you forgot.

Estimation Rules by Material

Concrete (1:2:4 mix): one cubic metre needs roughly 6.5 bags of cement (50 kg), 0.45 m³ of sand and 0.9 m³ of gravel. For a 10 m³ slab: ~65 bags cement, 4.5 m³ sand, 9 m³ gravel.

Steel reinforcement: thumb rules per m³ of concrete: slabs 80–100 kg, beams/columns 120–150 kg, footings 60–80 kg. A 10 m³ slab at 90 kg/m³ = 900 kg of rebar, plus 3–5% for laps and hooks.

Brickwork: ~50 bricks per m² of wall face per layer (for 225 × 112.5 × 75 mm bricks with 10 mm joints). A 100 m² single-brick wall = 100 × 50 × 2 × 1.05 ≈ 10,500 bricks. Mortar: one 25 kg bag of cement (mixed 1:4 with sand) per ~125 bricks.

Plaster (12 mm thick): at 1:4, one m² needs about 0.02 m³ of dry mortar mix: roughly 5–6 kg of cement (about 0.1 of a 50 kg bag) and 0.016 m³ of sand. For 200 m² of plaster: ~23 bags of cement (50 kg) and about 3.2 m³ of sand.

Paint: coverage ~10–12 m² per litre per coat on smooth plaster. Two coats on 200 m² = ~35–40 litres. Add 10% for texture and touch-ups.

Floor tiles: measure net area, add 7–10% for cutting wastage (15% for diagonal patterns or small rooms with many cuts).

Worked Example: A 12 × 10 ft Room (Walls Only)

Say you're pricing the brickwork and plaster for one room, 3.65 m × 3 m, walls 3 m high, single-brick, one door and one window:

  • Wall perimeter = 2 × (3.65 + 3) = 13.3 m; gross area = 13.3 × 3 = 39.9 m²
  • Openings: door 2.1 m² + window 1.5 m² = 3.6 m² → net 36.3 m²
  • Bricks = 36.3 × 50 × 2 × 1.05 ≈ 3,812 → 3,850 bricks
  • Cement for mortar = 3,812 ÷ 125 ≈ 31 bags of 25 kg
  • Plaster both sides: 36.3 × 2 = 72.6 m² → cement 72.6 × 0.11 ≈ 8 bags (50 kg), sand 72.6 × 0.016 ≈ 1.2 m³

Price each line from supplier quotes and you have a defensible wall cost in under ten minutes.

Wastage: What to Allow

  • Bricks/blocks: 5% (10% with many openings or curves)
  • Cement: 3–5% (it also has a shelf life: don't bulk-buy for a slow project)
  • Sand/gravel: 5–8% (spillage and uneven ground)
  • Steel: 3–5% for cutting and laps
  • Tiles: 7–10% (15% diagonal)
  • Paint: 10%

Getting Honest Prices

  • Three quotes minimum per major material, and make sure they're for the same brand and grade. "Cement" isn't one product.
  • Ask about bulk discounts: full truckloads of sand/gravel or 100+ cement bags can bring a worthwhile discount, so ask.
  • Include delivery: a cheap per-bag price with expensive delivery is the oldest trick in the book. Compare landed cost.
  • Lock prices in writing for the project duration where you can: steel and cement move monthly.
  • Check the calendar: prices spike in peak construction season. Buying steel or cement a month early, if you have dry storage, is legitimate savings.

Estimate It Free

Build the full BOQ in our material cost estimator: add a row per material with quantity and rate for an instant itemised total. For the quantity take-offs, the brick calculator handles walls and the lintel beam calculator works out rebar. The method above is also summarised in our brick calculation guide.