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Sampling Planner Calculator — IS 456 Cl 15.2.2 Significance

How many cubes per pour — and why 'one sample = 6 cubes' is more nuanced than the standard description suggests.

Why this calc exists

Sampling is the boring-but-critical first link in the acceptance chain. Too few cubes and a bad pour gets through; too many and the lab is overwhelmed. IS 456 Cl 15.2.2 gives a frequency band that depends on pour size and grade, but the language ('samples of 6 cubes each') doesn't tell you exactly how many samples YOUR pour needs.

The rules: 1 sample per 5 m³ for pours up to 30 m³; 1 sample per 10 m³ for 30–150 m³; for pours > 150 m³, the rate depends on the designer. For grades M30+, the designer may specify tighter sampling (some consultants double the frequency). The Sampling Planner calculator takes the pour volume, grade, and acceptance regime, and returns the cube count + sampling schedule.

Code references (with provenance)

  • IS 456 Cl 15.2.2 — Sampling rate: 1 sample per 5 m³ up to 30 m³, 1 per 10 m³ up to 150 m³, ≥ 1 per pour.
  • IS 456 Cl 15.2.2 Table 12 — Sample size: 6 cubes per sample (3 for 7-day, 3 for 28-day acceptance).
  • IS 456 Table 11 — Conformity criteria the samples feed into.
  • IS 456 Cl 16.1 — Strength acceptance criteria.

Worked: 60 m³ M25 pour

Cl 15.2.2 sampling rate: 60 m³ is in the 30–150 m³ band → 1 sample per 10 m³ = 6 samples.

Cubes per sample: 6 cubes × 6 samples = 36 cubes for this pour.

Lab workload: 36 cubes cast, 18 tested at 7 days, 18 at 28 days.

Cost: cube casting ~₹50 each (mould, casting, breaking) → ₹1,800 for this pour.

For an 8 m³ M25 pour: 1 sample per 5 m³ + ≥ 1 per pour → 2 samples → 12 cubes.

Field notes (what trips people up)

  • M30+ often doubles the rate. Some specifications (especially for infrastructure projects) require 2 samples per 10 m³ for M30 and above. Check the project spec.
  • One sample ≠ one cube. A sample is 6 cubes, period. Three go to 7-day, three to 28-day. Both subsets must independently meet acceptance.
  • Site lab capacity. A 36-cube pour is a half-day in the lab for casting + a full day for breaking. Plan lab capacity against the pour schedule.
  • Small pours are not exempt. Even a 3 m³ pour needs at least 1 sample = 6 cubes. The cube cost is a tiny fraction of the pour cost — don't skip.

Worked example

Site: 1,200 m³ podium slab pour, M40 grade.

Project spec: 2 samples per 50 m³ (double the IS 456 default for M40).

Samples: 1,200 / 50 × 2 = 48 samples.

Cubes: 48 × 6 = 288 cubes. Lab capacity: 9 cubes per testing day at 7 and 28 day → schedule lab time over 2 days per age.

FAQ

Why 6 cubes per sample?

3 for 7-day + 3 for 28-day. Each age subset is itself a mini-sample that the acceptance criterion applies to (mean of 3 ≥ fck + 4 for that age). If a single cube fails, the other 2 are still averaged — giving a fairer picture than 1 cube = 1 result.

What if I want to do only 7-day tests?

Possible but not advisable. 7-day is a leading indicator; 28-day is the contract acceptance. The cube cost is dominated by casting (~₹25), not testing (~₹25), so saving the 28-day set is false economy.

How does this fit with the acceptance criteria calc?

The sampling planner tells you HOW MANY cubes; the cube test statistics calc takes the cube results and checks against IS 456 Table 11. Use them sequentially: plan → cast → break → evaluate.

Related reading & tools

Indicative tool — not a substitute for engineering judgement. Results are computed from formulae and reference values cited to Indian Standards and other published codes as named on this page, using nominal default assumptions; clause and table references are given in good faith for study and preliminary checks. Always verify the current edition and amendments of any cited standard in the BIS catalogue before relying on it. Final mix proportions, structural checks and construction decisions must be confirmed by a qualified engineer against the project design brief, drawings and site conditions. ConcreteInfo accepts no liability for design, procurement or construction decisions taken on the basis of these tools. Code references & rights: Indian Standards are the property of the Bureau of Indian Standards (© BIS) — official PDFs at bis.gov.in. Shared in good faith as technical reference for the civil engineering fraternity — always verify against the official publication.