Row-to-Row vs. Hole-to-Hole Timing: The Most Important (and Most Confused) Concept in Blast Design
Timing is one of the most critical factors in surface blasting — and also one of the most confusing for students and young blasters. Getting it right separates a good blast from a great one. Getting it wrong leads to flyrock, poor fragmentation, excessive vibration, or uneven muckpiles.
The key is understanding that you are really working with two different timing scales that serve completely different purposes:
- Row-to-Row Timing (Macro Timing)
- Hole-to-Hole Timing (Micro Timing)
They are calculated differently, based on different geometries, and control different parts of the blast.
1. Row-to-Row Timing (Main Design Driver)
Purpose: Controls how the entire row of rock moves forward, manages energy release between rows, and creates relief for the next row.
Formula:
Example:
- Burden = 10 ft
- Row delay = 40 ms
Result: Excellent timing — well inside the recommended 3–8 ms/ft window.
What it controls:
- Rock movement and muckpile formation
- Overall fragmentation quality
- Safety (flyrock, airblast, and vibration)
NOTE: “Row timing controls how the blast moves.”
2. Hole-to-Hole Timing (Inside the Row)
Purpose: Controls how energy spreads along the row, creates stress wave interaction between holes, and ensures uniform breakage within that single row.
Important distinction: This is not based on burden. It is based on the spacing between holes in the same row.
Practical Rule of Thumb: Hole delay ≈ 1–3 ms per foot of spacing (most operations use ~2 ms/ft as a starting point).
Example:
- Spacing = 12 ft
- Using 2 ms/ft:
Recommended hole-to-hole delay: 20–25 ms
What it controls:
- Stress wave overlap
- Fragmentation uniformity inside the row
- Smooth, consistent break along the row
If the hole timing is wrong:
- Too fast → Holes act like one giant charge → high vibration, poor fragmentation
- Too slow → No interaction between holes → uneven breakage, large blocks left behind
Teaching line: “Hole timing controls how the row breaks.”
How Row and Hole Timing Work Together
REMEMBER: “First, we break the rock properly inside the row, then we move the broken rock row by row.”
Common Student Mistakes
Many students try to apply the ms/ft rule (designed for burden) to hole-to-hole timing. This is incorrect.
Correction: “ms/ft is for burden (row timing) — not for spacing (hole timing).”
“Hole timing controls the break. Row timing controls the movement. You need both to get a good blast.”
Put It Into Practice with PETS
To help students, engineers, and field crews master these concepts, we created PETS (Petr Explosives Timing Simulator)—a free online blast design calculator.
PETS lets you:
- Instantly calculate both row and hole timing
- Test different scenarios
- See whether your design falls inside safe and efficient ranges
- Troubleshoot problems before you load a single hole
It’s one of the best ways to move from theory to confident, practical blast design.
Want to master blast timing? Try the PETS Tool today at Petr Explosives Group and take your understanding to the next level. Whether you’re a student learning the fundamentals or an experienced blaster refining your designs, proper timing is the fastest way to safer, more efficient, and more profitable blasts.
This article is part of the Petr Explosives Group educational series on practical blasting science.
