Powder factor is the mass of explosive you load divided by the volume or tonnage of rock it breaks. Take the pounds in the holes, divide by the cubic yards in the shot, and you have lb/yd³. In metric, kilograms divided by cubic metres or tonnes.
Drill and blast engineers, shift supers and contractors use it two ways. Before the shot it sizes the pattern. After the shot it tells you whether the muckpile, the oversize and the digging rate came from the design or from something else.
The number only helps if you know which holes went in the numerator and which volume went in the denominator. The worked example below builds both from hole diameter, bench height and burden, so you can swap in your own inputs.
Worked example
Illustrative inputs, not field measurements: a 30 ft bench, 6.5 in holes loaded with bulk emulsion at a specific gravity of 1.2, an 18 ft burden, and short-interval delays between holes in a row (Ks = 1.2). Burden-based rules set subdrill at 0.3 x B and stemming at 0.7 x B.
| Step | Calculation | Result |
|---|---|---|
| Load factor (explosive mass per foot of hole) | 0.34 x 1.2 x 6.5^2 = 0.34 x 1.2 x 42.25 | 17.24 lb/ft |
| Subdrill | 0.3 x 18 ft | 5.4 ft |
| Stemming | 0.7 x 18 ft | 12.6 ft |
| Powder column | 30 + 5.4 - 12.6 | 22.8 ft |
| Charge per hole | 22.8 ft x 17.24 lb/ft | 393 lb |
| Spacing | 1.2 x 18 ft | 21.6 ft |
| Rock volume per hole | 18 x 21.6 x 30 = 11,664 ft³; 11,664 / 27 | 432 yd³ |
| Powder factor | 393 lb / 432 yd³ | 0.91 lb/yd³ |
Answer: Each hole carries 393 lb of explosive to break 432 yd³, a powder factor of 0.91 lb/yd³ (about 0.54 kg/m³), which sits in the SME High difficulty band of 0.75 to 1.25 lb/yd³.
Reading the number against the rock
A powder factor means little until you set it beside the rock. The SME Mining Reference Handbook, 2nd edition (CHAPTER 11: Blasting and Explosives, p. 284) groups general surface blasting by degree of difficulty, citing Dick et al. 1986:
| Difficulty | Powder factor, lb/yd³ |
|---|---|
| Low | 0.25-0.40 |
| Medium | 0.40-0.75 |
| High | 0.75-1.25 |
| Very high | 1.25-2.50 |
The 0.91 lb/yd³ bench lands in the High row. Suppose the face is soft, weathered ground that belongs in Medium. Then the design is overloaded, and you are paying for explosive that throws rock and shakes the neighbours without breaking it any better.
The same handbook ties powder factor to seismic velocity (p. 285, adapted from Heinen and Dimock 1976). Weathered limestone at 1,000 to 2,000 ft/s sits at 0.20 lb/yd³. Fresh limestone at 5,000 to 6,000 ft/s sits at 1.00, and massive jasperoid at 6,000 to 12,000 ft/s sits at 1.30. Faster rock needs more energy per yard.
What the equipment wants from the muckpile
The digging machine changes the target. The SME Mining Reference Handbook, 1st edition (Powder Factor for Underground Blasting, p. 209) lists typical surface coal figures by excavator and rock:
- Large dragline in shale: 0.5 to 0.8 lb/yd³.
- Shovel in shale: 0.6 to 0.9. Shovel in sandstone: 0.7 to 1.2.
- Front-end loader in sandstone: 0.8 to 1.3.
- Cast blasting in sandstone: 1.1 to 1.8.
Read the pattern. A dragline tolerates a coarser, looser pile, while a loader wants it finer. Cast blasting wants the most energy because the rock has to move as well as break.
The SME Mining Engineering Handbook, 3rd edition (7.3.4.2.1 Hole Diameter and Burden versus Face Height, p. 447) plots ranges by application: boulders, quarries, casting, mining and roads. Its axes run 0.5 to 6 kg/m³ and 1 to 5 lb/yd³. Use that figure for scale, and use your own shot history for the target.
Where a powder factor calculation goes wrong
Most bad numbers trace back to the inputs. Check these before you trust a result.
The volume basis. Use burden x spacing x bench height. Do not add the subdrill to the height. The subdrill is below grade, and that rock is not part of what you count as broken to grade.
Burden and spacing as drilled versus as fired. When holes fire in echelon or a V-cut, the geometric burden on the plan differs from the effective burden at the instant of firing. The SME Mining Reference Handbook, 2nd edition (p. 280) defines the effective burden as the distance from the hole to the nearest free face when the charge fires. It calls the empirical pattern rules first approximations that need adjustment to field conditions.
Which holes count. Count only loaded holes. Wet holes that took a different product, decked holes and holes lost to a collapse all change the numerator.
The direction of the ratio. The SME Mining Engineering Handbook, 3rd edition (7.3.4.2.6 Decking, p. 450) says powder factor can be worked per hole or per whole shot. It can also be quoted as weight of explosive per unit weight of rock. When you read someone else's figure, confirm whether it is lb/yd³, kg/m³ or kg/tonne before comparing it with yours.
Mixed rock. Weathered cap over fresh rock on the same bench has two different targets. One average hides both problems.
Cross-checking the pattern geometry
The worked example used burden-based rules: subdrill at 0.3 x B and stemming at 0.7 x B. The SME Mining Reference Handbook, 1st edition (Method Based on Known Powder Factor, p. 207) also gives a diameter-based set, where stemming T = 1.7 x De and subdrill J = 0.5 x De, with De in inches and the results in feet.
For a 6.5 in hole, that gives 11.1 ft of stemming and 3.3 ft of subdrill. The burden-based set gave 12.6 ft and 5.4 ft. The two sets disagree by a foot or two, which is normal for empirical rules. Pick one set, keep it across the site, and adjust from what the muckpile and the back of the shot tell you.
The spacing ratio matters just as much. The same page gives Ks = 2.0 for simultaneous initiation, 1.0 for long delay intervals between holes in a row, and 1.2 to 1.8 for short intervals. Change Ks and the volume per hole changes, so the powder factor moves even though the charge did not.
Holding the load under the delay limit
The powder factor says nothing about how much explosive fires at once. Two shots can have the same lb/yd³ and very different vibration, because one fires 393 lb per delay and the other fires 1,200.
When the limit is tight, decking is the standard tool. The SME Mining Engineering Handbook, 3rd edition (7.3.4.2.6 Decking, p. 450) sets the deck material at 6 times the hole diameter for dry holes and 12 times for wet holes. For a 6.5 in hole, that is 39 in of stemming between decks when dry and 78 in when wet. A delay, typically 25 ms, separates the decks so the ground sees two small thumps instead of one large one.
Decking also handles voids and mud seams. Air decks cut the load without piling on stemming that leaves blocky ground at the top of the muckpile. Every deck lowers the effective powder factor in that hole, so recalculate after you deck.
Surface metal and nonmetal operations work under 30 CFR Part 56, Subpart E on explosives. Read it for loading, stemming and misfire requirements before you change a design.
Square, staggered and echelon layouts
The drilled grid and the firing order are separate decisions, and both feed the number.
- Square grid. Spacing equals burden. Easy to lay out and drill, but each hole breaks to a free face only when the delay allows it.
- Rectangular grid. Spacing exceeds burden, usually with Ks between 1.2 and 1.8 when holes fire at short intervals.
- Staggered grid. Alternate rows are offset by half the spacing. Same volume per hole, more even energy distribution.
- Row-by-row, echelon and V-cut firing. These change the effective burden, as the p. 280 definition above explains.
Whatever you drill, calculate the volume from the grid you actually laid out and the effective geometry you actually fired.
Development rounds run on a different scale
A tunnel heading has one free face and no bench. Every hole fights confinement, so powder factor climbs. The SME Mining Reference Handbook, 1st edition (Controlled Blasting, p. 210, Figure 11.3) plots an inverse relationship between heading area and required powder factor for 1.25 to 2 in holes in granite and basalt, sandstone and shale, and limestone. The y-axis runs from 0 to 12 lb/yd³. Small headings need much more explosive per yard than large ones.
The arithmetic is the same, and the volume is the heading area times the advance. Assume a 12 ft by 14 ft heading, which is 168 ft², with a 10 ft pull:
1. Volume: 168 x 10 = 1,680 ft³. Divide by 27 to get 62.2 yd³. 2. Assume the round carries 250 lb. Powder factor: 250 / 62.2 = 4.0 lb/yd³.
Those inputs are illustrative. Use the advance you actually measured, not the drilled depth. A round that pulls 8.5 ft of a 10 ft drill has a higher true powder factor than the plan says. To size a full round from hole count, burden and charge length, the underground round charge calculator does the bookkeeping.
Checking the number after the shot
A calculated powder factor is a prediction. Close the loop with what happened:
1. Record the explosive actually loaded, hole by hole, not the plan. 2. Record the volume actually broken to grade. 3. Note fragmentation, oversize, back-break, toe and diggability. 4. Move the next shot's target up or down by a small step and note why.
Over a few shots on one face you build a local table that beats any handbook range. The handbook gets you to a defensible first design, and your own records get you the rest of the way.
Tying the number to cost
Explosive is one input. The holes cost more than most crews expect, and the next shot's pattern changes how many you drill. Once you know yards per hole, you can work out what each drilled meter really costs and compare a wider pattern with a heavier load on total cost, not on lb/yd³ alone.
Keep the loading records too. When a client disputes quantities or blames a blast for a delay, shift-by-shift records of what was loaded and what was dug settle it. That is the same discipline behind winning or losing a back-charge dispute with daily reports.
Run your own numbers
The underground round charge calculator does this arithmetic and shows its working, so you don't have to run it by hand each time. The rest of the set is on the mining calculators page.
Common questions
What is the charge per delay in blasting?
Charge per delay is the mass of explosive that fires at the same instant, or within one delay interval, summed across every hole and deck sharing that delay. In the example, one 393 lb hole per delay means 393 lb; two holes sharing a delay means 786 lb. Stemming decks with a 25 ms delay between them split one column into two smaller charges.
How to calculate blast load?
Work per hole, then multiply by the hole count. Load factor is 0.34 x specific gravity x diameter squared, in lb/ft. Powder column is bench height plus subdrill minus stemming. In the example, 22.8 ft x 17.24 lb/ft gives 393 lb per hole. Divide the shot total by the rock volume to get powder factor.
What are the different types of blasting patterns used in opencast mines?
Opencast blasts are drilled on square, rectangular or staggered grids, then fired row by row, in echelon or in a V-cut. The delay layout sets the spacing-to-burden ratio: SME gives Ks = 2.0 for simultaneous firing, 1.0 for long delays and 1.2 to 1.8 for short delays between holes in a row.
Getting this off paper
Most contractors already know these numbers. The cost is in re-keying them into a report, then a timesheet, then an invoice. If that is where your week goes, book a 20-minute call and we will look at your actual paperwork.
