25 Maths Formulas for the OC and Selective Tests

OC and Selective Maths is mostly reasoning, not formula recall — but these 25 formulas and facts come up again and again. Worth knowing by heart.

OC (Year 4) leans on the shape, number and measurement basics; Selective (Year 6) uses the lot, including circles and surface area.

Shapes — area & perimeter

Rectangle area — length × width (6 × 4 = 24 cm²)

Rectangle perimeter — 2 × (length + width) (2 × (6 + 4) = 20 cm)

Square area — side × side (5 × 5 = 25 cm²)

Square perimeter — 4 × side (4 × 5 = 20 cm)

Triangle area — ½ × base × height (½ × 8 × 5 = 20 cm²)

Parallelogram area — base × height (6 × 4 = 24 cm²)

Circle circumference — π × diameter (π ≈ 3.14) (3.14 × 10 ≈ 31.4 cm)

Circle area — π × radius × radius (3.14 × 5 × 5 ≈ 78.5 cm²)

Solids — volume & surface

Cuboid volume — length × width × height (3 × 2 × 2 = 12 cm³)

10.

Cube volume — side × side × side (4 × 4 × 4 = 64 cm³)

11.

Cuboid surface area — 2 × (lw + lh + wh) (2 × (6 + 6 + 4) = 32 cm²)

Angles

12.

Angles in a triangle — add up to 180° (90° + 60° + ? → ? = 30°)

13.

Angles on a straight line — add up to 180° (130° + ? = 180° → ? = 50°)

14.

Angles around a point — add up to 360° (120° + 150° + ? = 360° → ? = 90°)

15.

Angles in a quadrilateral — add up to 360° (90° + 90° + 100° + ? = 360° → ? = 80°)

Number, fractions & percentages

16.

Average (mean) — total ÷ how many ((4 + 6 + 8) ÷ 3 = 6)

17.

Fraction of an amount — amount ÷ denominator × numerator (¾ of 20 = 20 ÷ 4 × 3 = 15)

18.

Percentage of an amount — (percentage ÷ 100) × amount (25% of 80 = 20)

19.

Fraction → percentage — (numerator ÷ denominator) × 100 (3 ÷ 4 × 100 = 75%)

20.

Probability — favourable outcomes ÷ total outcomes (2 red of 5 = 2/5)

Money, rates & measurement

21.

Speed — distance ÷ time (distance = speed × time) (120 km ÷ 2 h = 60 km/h)

22.

Total cost — price per item × number of items ($3 × 4 = $12)

23.

Change — amount paid − total cost ($20 − $12 = $8)

24.

Time — 1 h = 60 min · 1 min = 60 s · 1 day = 24 h (2 h = 120 min)

25.

Metric units — 1 km = 1000 m · 1 m = 100 cm · 1 cm = 10 mm · 1 kg = 1000 g · 1 L = 1000 mL (2 km = 2000 m)

How to actually use them

Learn what each one means, not just the letters — exams hide them in word problems.
Always write the units (cm, cm², km/h) — they catch mistakes.
Area vs perimeter: inside the shape (area) or around it (perimeter)?

Practise these in real questions — see our OC Maths practice questions, or start OC and Selective practice with every question explained.

PrepPath is an independent educational platform and is not affiliated with ACARA or the NSW Department of Education. This guide is general information only — always check the official ACARA (nap.edu.au) or NSW Department of Education websites for current dates and rules.