Bench Blast Design Calculator
Enter hole diameter, bench height, rock density and explosive density to get a first-trial burden (Konya or Ash), spacing, stemming (0.7 B), subdrill (0.3 B), hole depth, loading density, ANFO charge per hole and powder factor. Rules of thumb for trained blasters; follow your regulations.
Konya's first-trial burden is B (ft) = (2·SGe/SGr + 1.5) × De (in); stemming ≈ 0.7 B, subdrill ≈ 0.3 B, delayed spacing (H + 7B)/8 or 1.4 B, and the ANFO charge per hole is π/4·d²·ρ times the hole depth minus stemming.
Use the Bench Blast Design Calculator
For trained blasting professionals — follow your regulations
These are first-trial rules of thumb, not a blast plan. Blasting must be designed and carried out by a licensed or certified blaster under local law, using the explosive manufacturer's data, site geology and your vibration, air-blast and flyrock limits.
Blast Design Inputs
Bulk-loaded explosive fills the hole; for cartridges enter the cartridge diameter.
Limestone 2.3–2.7, granite 2.6–2.7, basalt 2.8–3.0 t/m³ (specific gravity).
Use the density and RWS on your product's data sheet when you have them.
≈ 25 surface, 20 underground (ANFO, rock ≈ 2.5 t/m³).
1 to 1.3.
Sample Scenarios
Enter parameters and click Calculate Blast Parameters.
Blast Geometry
Explosive Load & Powder Factor
Blast Hole Cross-Section (schematic)
Explosive loading density, kg per metre of hole
kg/m = π/4 × d² × ρ = 0.000785 × ρ (g/cm³) × d² (mm). Multiply by the explosive column length for the charge per hole.
| Hole diameter | 0.82 g/cm³ | 0.85 g/cm³ | 1.00 g/cm³ | 1.15 g/cm³ | 1.25 g/cm³ |
|---|---|---|---|---|---|
| 76 mm (2.99 in) | 3.72 | 3.86 | 4.54 | 5.22 | 5.67 |
| 89 mm (3.50 in) | 5.10 | 5.29 | 6.22 | 7.15 | 7.78 |
| 102 mm (4.02 in) | 6.70 | 6.95 | 8.17 | 9.40 | 10.21 |
| 115 mm (4.53 in) | 8.52 | 8.83 | 10.39 | 11.94 | 12.98 |
| 127 mm (5.00 in) | 10.39 | 10.77 | 12.67 | 14.57 | 15.83 |
| 152 mm (5.98 in) | 14.88 | 15.42 | 18.15 | 20.87 | 22.68 |
| 165 mm (6.50 in) | 17.53 | 18.18 | 21.38 | 24.59 | 26.73 |
| 200 mm (7.87 in) | 25.76 | 26.70 | 31.42 | 36.13 | 39.27 |
| 229 mm (9.02 in) | 33.77 | 35.01 | 41.19 | 47.37 | 51.48 |
| 251 mm (9.88 in) | 40.57 | 42.06 | 49.48 | 56.90 | 61.85 |
| 270 mm (10.63 in) | 46.95 | 48.67 | 57.26 | 65.84 | 71.57 |
| 311 mm (12.24 in) | 62.29 | 64.57 | 75.96 | 87.36 | 94.96 |
Imperial: lb/ft = 0.3405 × SG × D² (in). ANFO poured density is 0.82–0.85 g/cm³; heavy ANFO blends 0.95–1.35, bulk emulsions 1.20–1.35 and packaged emulsions and water gels 1.05–1.28 g/cm³ (Mirabelli, Explosive Products).
Rules of thumb used (first-trial values)
- Konya burden: B (ft) = (2·SGe/SGr + 1.5) × De (in). Stemming T = 0.7 B, subdrill J = 0.3 B. Spacing with delays: S = (H + 7B)/8 if 1 < H/B < 4, S = 1.4 B if H/B ≥ 4; field spacing within ±15% of the value. Stiffness ratio H/B ≈ 1 poor, 2 fair, 3 good, ≥ 4 excellent.
- Ash ratios: B = K_B × D with K_B ≈ 25 (surface) or 20 (underground) for standard ANFO in rock of about 2.5 t/m³; K_B for another explosive = K_B × √(relative bulk strength); S = K_S × B with K_S 1–1.3; T = 0.7 B. Here relative bulk strength = RWS/100 × density/0.85.
- Powder factor = explosive per hole ÷ rock under the pattern area to bench depth (B × S × H).
Sources: Konya formulas and worked examples in University of Maryland ENCE 420 notes, Blasting Rock (after Peurifoy, Construction Planning, Equipment, and Methods); Ash ratios in Penn State MNG 230, Bench Blasting. Instantaneous (non-delayed) firing uses different spacing rules and is not covered.
Quick answer
A first-trial bench blast design starts from the burden. Konya's rule of thumb: B (ft) = (2·SGe/SGr + 1.5) × De (in), where SGe and SGr are the explosive and rock specific gravities and De the explosive (hole) diameter. Ash's rule: B = K_B × D with K_B ≈ 25 for surface blasts with ANFO in rock of about 2.5 t/m³. Then stemming T ≈ 0.7 B, subdrill J ≈ 0.3 B, hole depth L = H + J, and for delayed holes spacing S = (H + 7B)/8 when H/B < 4 or 1.4 B when H/B ≥ 4. Loading density = π/4 × d² × explosive density (0.3405 × SGe × De² lb/ft); charge per hole = loading density × (L − T); powder factor = charge ÷ (B × S × H). These are starting points for a trial shot, for trained professionals only: follow your regulations and the explosive maker's data.
Examples
| Input | Result (Konya, delayed) |
|---|---|
| 3 in hole, 2.5 in explosive, SGe 1.2, granite SGr 2.75, H = 20 ft | B = (2 × 1.2/2.75 + 1.5) × 2.5 = 5.93 ft |
| Stiffness ratio H/B = 3.37 (< 4) | S = (20 + 7 × 5.93)/8 = 7.69 ft |
| T = 0.7 B, J = 0.3 B | T = 4.15 ft, J = 1.78 ft, L = 21.78 ft |
| ANFO 0.85 g/cm³ in a 200 mm hole | Loading density = 26.7 kg/m |
Enter your hole diameter, bench height, rock density and explosive to get a first-trial pattern.
Summary
This calculator produces a first-trial surface bench blast pattern from published rules of thumb. Burden comes from Konya's empirical formula B = (2·SGe/SGr + 1.5)·De (feet, with De in inches), as taught in construction-engineering courses, or from Ash's ratio B = K_B·D (K_B about 25 for surface blasting with ANFO in rock near 2.5 t/m³, adjusted by the square root of the explosive's relative bulk strength). Stemming is 0.7 B, subdrill 0.3 B, and spacing for delay-fired holes follows Konya's stiffness-ratio rule (or Ash's K_S × B). The explosive column is the hole depth minus the stemming; the charge per hole uses the loading density π/4·d²·ρ, and the powder factor divides that charge by the rock under the pattern (B × S × H). Results are estimates for trained blasters and engineers, not a blast plan.
How it works
- Choose SI or imperial units and the burden method (Konya or Ash).
- Enter hole diameter, bench height and rock density, and pick the explosive (or type its density from the data sheet).
- Burden: Konya B = (2·SGe/SGr + 1.5)·De, or Ash B = K_B·D·√(RBS).
- Spacing for delayed initiation: S = (H + 7B)/8 if H/B < 4, else 1.4 B (Konya); or S = K_S·B (Ash). Stemming T = 0.7 B, subdrill J = 0.3 B (editable).
- Hole depth L = H + J, explosive column Lc = L − T, charge = loading density × Lc.
- Powder factor = charge ÷ (B × S × H), in kg/m³ and kg/t (lb/yd³ and lb/ton).
Use cases
- A first-trial pattern for a quarry, open-pit or construction bench before test blasts.
- Checking a contractor's proposed burden, spacing and stemming against standard rules of thumb.
- Estimating ANFO per hole and explosive consumption for a pattern.
- Seeing how hole diameter, bench height and explosive density change burden, spacing and powder factor.
- Coursework and exam practice in rock blasting and construction methods.