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Specific Charge Calculator

Computes specific charge (powder factor) in kg/m³ — and kg/t with a rock density — from charge per hole and burden × spacing × bench height, a whole-blast total, or several holes with your own volumes.

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How to use this calculator

Inputs

Powder factor in kg/m³ (and kg/t with a density) from charge per hole and burden × spacing × bench height, a whole-blast total, or several holes.

Charge
Pattern

Crest to toe of the bench — not the drilled hole length, which also includes subdrill.

More options
Hole

Not added to the volume: the attributable volume uses bench height.

Per tonne

Optional — adds kg/t. Use a measured or sourced density for this rock.

Enter the values and press Calculate.

Or load an example to see a worked result.

Bench geometry and charge basis
Not to scale
Bench geometry and charge basisSection through a blast bench: crest, free face and toe, a blast hole set back by the burden, bench height from crest to toe, and subdrill below the toe.HoleCrestToeFreefaceHBSubdrillHole length ≠ HV = B × S × HS into page
Bench height H (not entered), measured crest to toe. Burden B (not entered) from the hole to the free face. A drilled hole usually continues below the toe (subdrill), so hole length is longer than bench height.

Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.

How to use this calculator

  1. Choose the basis: per hole (burden × spacing × bench height), a whole blast with your own blasted volume, or several holes with their own attributable volumes.
  2. Enter the charge and the dimensions or volume. Use the bench height (crest to toe), not the drilled hole length.
  3. Optionally add the in-situ rock density to get kg/t as well as kg/m³.

How it works

Per hole: attributable volume V = B × S × H, and specific charge q = charge ÷ V in kg/m³. Subdrill is not added to the volume.

kg/t = (kg/m³) ÷ density in t/m³ — a different unit, not a second answer. Whole-blast mode divides the total charge by the volume you enter; no edge or boundary-hole correction is guessed.

No good/bad bands are applied: powder factor alone does not predict fragmentation, throw or vibration. Compare it with the limits in your approved blast design.

Worked example

90 kg per hole, 3 × 3.5 × 10 m. V = 3 × 3.5 × 10 = 105 m³, so q = 90 ÷ 105 = 0.857 kg/m³ (0.857142857…). With an in-situ rock density of 2.5 t/m³ that is 0.857 ÷ 2.5 = 0.343 kg/t.

Frequently asked questions

kg/m³ or kg/t?

kg/m³ is per volume of rock in place; kg/t is per mass, so it needs the rock density: kg/t = kg/m³ ÷ t/m³. Use the unit your blast design specifies — the two numbers are not interchangeable.

Does subdrill enter volume?

Not here. The ideal attributable volume uses bench height (crest to toe). Subdrill is drilled below the toe to break the floor; adding it would understate the specific charge unless your design model says otherwise.

Does higher charge mean better fragmentation?

Not by itself. Fragmentation also depends on rock mass, geometry, timing, explosive and confinement. A specific charge is a design quantity to compare with your approved design, not a quality score.

Is specific charge the same as powder factor?

Yes — both express explosive per unit of rock. Check whether a figure is per m³ or per tonne before comparing.

Related tools

Explore more in Mining, Quarry, Earthworks, Drill & Blast.

Cite this tool

Zerdly. (2026). Specific Charge Calculator. Retrieved from https://www.zerdly.com/tools/specific-charge-calculator/

Tip: Enter any known values to calculate the remaining results.

Designed to run locally in your browser. Tool inputs are not intentionally stored by Zerdly unless stated otherwise.