Powder Factor in Blasting: Formula, Worked Example and Normal Ranges

A hard-rock quarry bench usually takes 0.7 to 0.8 kg of explosive to break each cubic metre of rock. That ratio is the powder factor in blasting: the weight of explosive used, divided by the volume or the weight of rock the blast breaks.

It is the one number that tells a quarry owner whether a blast was loaded about right. Too low and the face comes down in boulders that the excavator cannot lift and the crusher cannot take. Too high and money goes up in smoke, with flyrock and ground vibration as the price.

This guide gives the formula, works one full calculation for a granite bench from hole size to kg per tonne, lists the normal ranges by rock type, shows how to run the sum backwards to set burden and spacing, and explains what the broken heap tells you after the blast. It carries no rupee figures and it is not a licence to charge holes: that work belongs to a licensed blaster.

What is powder factor in blasting?

Powder factor is explosive weight divided by rock broken. “Powder” is the old blasting word for explosive, from the days of gunpowder, and the name has stuck even though quarries now load ANFO (ammonium nitrate mixed with diesel) or emulsion.

It is written two ways:

• kg per cubic metre (kg/m³), rock measured in the solid, before it is broken. This is the form used for blast design.
• kg per tonne (kg/t), rock measured by weight. This is handier when the plant counts output in tonnes.

Older American books use pounds per cubic yard or pounds per ton. One kg per cubic metre equals 1.69 pounds per cubic yard.

Penn State University’s introductory mineral engineering course describes it as a measure of how efficient a blast is. Mines that keep it on record can compare one blast with the next, and a new pit can borrow figures from a similar one to make its first design.

What is the powder factor formula?

The formula breaks into a few short steps. Work out the explosive in one hole, work out the rock that one hole breaks, then divide.

Two terms need a word. Burden is the distance from a row of holes to the open face in front of it, or to the next row. Spacing is the distance between two holes in the same row. Burden times spacing is the patch of ground one hole is responsible for, and that patch times the bench height is the block of rock it has to break.

For a whole blast the sum is the same. Add up the explosive in every hole and divide by the total volume: number of holes × burden × spacing × bench height.

How to calculate powder factor: a worked quarry bench example

Take a granite quarry drilling 115 mm holes on a 9 m bench, loaded with ANFO. Everything below comes from the rules of thumb in Dyno Nobel’s Blasting and Explosives Quick Reference Guide, an explosives maker’s field handbook that says plainly these rules give a first estimate in the absence of better data.

Set the pattern. The guide’s rules, worked for a 115 mm hole:

• Bench height in metres should be at least the hole diameter in mm ÷ 15. 115 ÷ 15 = 7.7 m, so a 9 m bench is fine.
• Burden is 25 to 40 times the hole diameter: 2.9 to 4.6 m. Start at 3.0 m.
• Spacing is about 1.15 times the burden: 3.45 m. Round to 3.5 m.
• Subdrill is 3 to 15 times the diameter: 0.35 to 1.7 m. Take 1.0 m, so the hole is 10 m deep.
• Stemming is at least 20 times the diameter (2.3 m), or 0.7 to 1.2 times the burden (2.1 to 3.6 m). Take 2.5 m.

Penn State’s course reaches the same burden from the other direction: 25 times the hole diameter for a surface bench with standard ANFO, which gives 2.9 m.

Do the sum.

The ANFO figure of about 840 kg per cubic metre is its bulk density. The granite density of 2.72 t/m³ is the figure in the Dyno Nobel guide’s table of rock properties.

Read the answer. 0.69 kg/m³ sits just under the 0.7 to 0.8 band that the same guide gives for hard rock. On granite that is a warning of a coarse heap with some boulders. The fix is a slightly tighter pattern: at 2.9 m × 3.3 m the hole breaks 86 m³ (234 tonnes), and the same 65 kg now gives 0.76 kg/m³, or 0.28 kg/t. The bench height to burden ratio is 3.1, inside the 2 to 3.5 range the guide links with good fragmentation.

What is a normal powder factor for hard, medium and soft rock?

The most quoted starting bands come from the Dyno Nobel guide. The Health and Safety Authority of Ireland, the quarry regulator there, reproduces them in its quarry blasting safety material and adds the rock names for each class.

In Indian terms, that puts granite and basalt quarries at the top of the table and hard limestone pits for cement in the middle. Treat the bands as a starting point. The HSA material advises comparing the planned charge with previous successful blasts on the same face, and in practice a quarry settles its own figure after a few trial blasts.

The bands are for ANFO or explosives of similar strength. Strength by weight is measured against ANFO, taken as 100. The HSA material puts emulsion explosives at 83 to 125 per cent of ANFO, so a kilogram of one explosive is not always worth a kilogram of another. The Dyno Nobel guide handles this with an energy factor: powder factor × the explosive’s relative weight strength. When you change explosive, compare energy factors, not kilograms.

Perimeter blasting, used to leave a clean final wall, is measured differently: in kg per square metre of face rather than per cubic metre. The guide gives 0.5 kg per square metre for presplitting as its rule of thumb.

Bench height or hole length: which volume goes in the formula?

This is where two powder factors for the same blast can disagree, so it is worth checking before you compare your figure with anyone else’s.

• The Dyno Nobel guide divides by burden × spacing × bench height. The subdrill below the floor is left out, because that rock belongs to the next bench down. Our example gives 0.69 kg/m³.
• The Penn State course divides by burden × spacing × hole length, subdrill included. The same blast gives 0.62 kg/m³.
• The HSA material also describes an in-hole powder factor, which counts only the face height that is actually charged (face height minus top stemming). On the same blast this reads 0.96 kg/m³, because the rock beside the stemming is left out.

None of these is wrong. They answer slightly different questions. What matters is that a quarry picks one basis, writes it next to the figure, and keeps to it, so that this month’s blasts are compared with last month’s on the same terms. The ranges in Table 3 are on the total-volume basis.

How do you work backwards from a powder factor to burden and spacing?

The formula also runs in reverse, and this is where it earns its place in blast design. If you know the powder factor that suits your rock, you can find the pattern.

1. Work out the explosive per hole from the hole size and charge length. In the example, 65 kg.
2. Divide by the target powder factor to get the rock each hole should break. 65 ÷ 0.75 = 87 m³.
3. Divide by the bench height to get burden × spacing. 87 ÷ 9 = 9.7 m².
4. Choose a spacing to burden ratio and solve. With spacing = 1.15 × burden, burden = √(9.7 ÷ 1.15) = 2.9 m, and spacing = 3.3 m.

That is the same 2.9 m × 3.3 m pattern the forward calculation arrived at, which is the point of doing it both ways. The Penn State course recommends exactly this cross-check: lay the pattern out from the hole diameter, then test it against a powder factor from a similar mine.

The explosive’s density changes step 1, and with it the whole pattern. The guide’s loading table shows how much.

A denser explosive puts half as much again into every metre of the same hole. Keep the same pattern and the powder factor jumps; keep the same powder factor and the pattern can open up.

What pushes the powder factor up or down?

Rock. Hard, dense, tough rock needs more explosive per cubic metre. That is the whole of Table 3. Joints and cracks also matter: a weak band or a cavity in front of a hole lets the charge escape that way.

The size you want. A crusher with a small feed opening needs finer rock off the face, and finer rock costs more explosive. A pit sending rock to a big primary jaw crusher can run a lower powder factor than one feeding a small mobile plant. The guide to jaw, cone and VSI crushers explains what each stage can accept.

Hole size. Bigger holes allow a wider pattern, and the powder factor can look the same while the rock comes out coarser, because the explosive sits in fewer, bigger columns. The Penn State course puts it bluntly: one giant hole packed with explosive might give a good-looking powder factor and a bad blast. How evenly the explosive is spread through the rock decides fragmentation, not the total alone.

Wet holes. ANFO dissolves in water and cannot be loaded into holes with standing water, so wet benches move to water-resistant emulsions. Emulsions are denser, so the kilograms per hole change even when the pattern does not.

Bench height. On a low bench the stemming and subdrill take a bigger share of every hole, so the figure behaves differently from a tall one. The guide to bench height in opencast mining covers how benches are set.

What does the muckpile tell you about the powder factor?

The heap of broken rock left after a blast, the muckpile, is the report card. Read it before the excavator starts digging.

Flyrock is the dangerous one. The HSA’s list of causes is worth knowing by heart: the explosive column brought too high up the hole so the top stemming ends up shorter than the burden; bulk explosive running into a cavity or crack; two holes drifting together while drilling; an angled hole whose bottom ends up closer to the face than planned; rock that has fallen out of the face since it was surveyed; and simply more explosive than the rock type needs. Every one of them is a local overcharge, a spot where the powder factor is far higher than the average for the blast.

Boulders cost money in a different way. Each one has to be broken again before it can be loaded or crushed, usually by an excavator fitted with a hydraulic rock breaker. The HSA material notes that the usual cure for a thin burden at the top of a face, more stemming, leaves exactly this oversize behind. The guide to overburden removal makes the same point from the digging side: better fragmentation is often the cheapest output gain on a site.

Which machines depend on getting the powder factor right?

In a quarry, every machine after the drill works on whatever the blast leaves. The loading excavator sets the pit’s daily output, and a well-broken, loose muckpile lets it fill its bucket in fewer passes. A blocky one slows every cycle and wears the bucket teeth and undercarriage faster. The guide to quarry equipment walks through each stage from drill to stockpile.

Face loading in a stone quarry is usually done by large excavators such as the Tata Hitachi EX 350LC Prime, and in bigger pits by mining-class machines such as the SANY SY580C-10HD and Tata Hitachi EX 1200V.

If you are planning a quarry fleet, compare mining excavators and their specifications, and see which rock breaker suits each excavator for secondary breaking.

Who is allowed to charge and fire the holes in India?

Not the quarry owner and not the excavator operator. IS 4081 : 2013, the Indian Standard code of safety for blasting and related drilling, defines the blaster as the person assigned to load and fire the explosives who holds statutory approval or a licence to handle them (clause 3.1). Blasting needs the permission of the engineer-in-charge and stays in the charge of a competent supervisor and blaster (clause 8.1). On a mine, the regulations administered by the Directorate General of Mines Safety (DGMS) apply as well; the standard itself says all statutory provisions must be followed (clause 4).

Two clauses matter for powder factor directly. Clause 8.14 requires the blaster to keep an accurate record of the explosives and blasting supplies used in every blast. That record, divided by the rock the blast produced, is your actual powder factor. And clause 12.3 sets out warning rules for regular open blasting, including barriers on every approach at least 400 m away, closed 10 minutes before firing.

Explosives reach a quarry through licensed dealers and blasting contractors; the post on explosives companies in India lists who makes them. For the wider picture of how opencast pits, underground mines and quarries differ, start with the guide to types of mines, and find every topic under mining.

Prices, specifications and features are indicative, vary by variant, location and date, and should always be confirmed with the official OEM or authorised dealer before any purchase decision. DesiMachines is not liable for decisions taken on the basis of information that may have changed after publication. This guide explains blast arithmetic only; charging and firing explosives is work for a licensed blaster under the rules that apply to your site.

References: Dyno Nobel, Blasting and Explosives Quick Reference Guide (2010): blast design formulas, rules of thumb, typical powder factors, loading density table, rock properties table. IS 4081 : 2013 (reaffirmed 2018), Blasting and Related Drilling Operations: Code of Safety, clauses 3.1, 4, 8.1, 8.14 and 12.3. Health and Safety Authority (Ireland), “Determining Minimum Burdens for Quarry Blasting”, All Island Quarry Safety Conference: causes of flyrock, in-hole powder factor, explosive strength against ANFO, rock examples by class. Penn State University, MNG 230 Introduction to Mineral Engineering, lesson 8.3.1 Bench Blasting: powder factor definition and the reverse calculation. Wikipedia, ANFO: bulk density and water solubility.