How to Calculate Powder Factor

HOW-TO · AUG 2026 · 3 MIN READ

Powder factor is blasting's exchange rate: pounds of explosive per unit of rock broken. It's the first number a blaster quotes, the first number an auditor checks, and a two-minute calculation that people fumble in exactly two predictable places. Here's the clean method.

01Compute the rock per hole

Each production hole owns a block of rock: burden × spacing × bench height. Pattern at 12 ft burden, 14 ft spacing, 40 ft bench: 12 × 14 × 40 = 6,720 ft³. Divide by 27 for cubic yards: 248.9 CY. Note: bench height, not hole depth — subdrill breaks floor, it doesn't add pattern volume.

02Compute the explosive per hole

Column charge = loaded length × loading density. Loaded length = hole depth − stemming = (40 + 4 subdrill) − 12 stemming = 32 ft. Loading density for the product and diameter comes from de = 0.3405 × SG × d² (lb/ft, d in inches): a 6½" hole of ANFO at 0.85 SG loads about 12.2 lb/ft. 32 ft × 12.2 = ~390 lb per hole.

03Divide and state the units

PF = 390 lb ÷ 248.9 CY = 1.57 lb/CY. Say the units out loud: lb/CY and lb/ton are both called 'powder factor' and differ by the rock density. At 2.2 tons/CY in-place, 1.57 lb/CY = 0.71 lb/ton. Metric crews run kg/m³ — 1 lb/CY ≈ 0.59 kg/m³.

04Check against the normal ranges

Typical surface work: soft limestone 0.6–1.0 lb/CY, medium rock 0.9–1.3, hard granite and trap 1.2–1.8. Outside the range isn't automatically wrong — but it demands a reason (cast blasting, oversize crusher, terrible ground) before it demands a signature.

05Tune with the pattern, not just the powder

PF rises by shrinking burden/spacing or richening the load; it falls the opposite way. The cheapest ton usually comes from the widest pattern that still breaks to spec — creep B and S out while fragmentation holds, and PF drifts down with your cost per ton.

The TrapTwo unit crimes cause 90% of wrong answers: forgetting the ÷27 (leaving PF in lb/ft³, which looks absurdly small) and mixing lb/CY with lb/ton. On the PE exam, the unconverted number is always among the answer choices — it's not a coincidence, it's bait.

Rule of ThumbPF = (loaded ft × lb/ft) ÷ (B×S×H÷27). And memorize de = 0.3405 × SG × d² — it unlocks half the blasting problems ever written.

Drill the exam version in the free Study Vault, or feel the consequences in Blast Master — where a lazy powder factor puts rocks over the highwall.

Studying for the PE Mining exam?
The free Study Vault has exam-style problems with worked solutions, trap warnings, and rules of thumb.

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