ROM Grade Calculator
The Run-of-Mine (ROM) grade of a blend is the mass-weighted average of its parts, not a simple average.
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How to use this calculator
- Enter the tonnes and grade for each of the two streams you are blending — keep BOTH grades in the same unit (both g/t, or both %).
- To model dilution, enter the ore as stream 1 and the waste/dilution tonnage as stream 2 with a grade of 0.
- Read the blended ROM grade, the total blended tonnes and the total contained metal (grade-tonnes).
How it works
The Run-of-Mine (ROM) grade of a blend is the mass-weighted average of its parts, not a simple average. The tool computes ROM grade = (t1 × g1 + t2 × g2) ÷ (t1 + t2), where t is tonnes and g is grade in a consistent unit. The numerator is the total contained metal expressed as grade-tonnes; dividing by total tonnes returns the average grade delivered to the ROM pad or crusher.
Because it is mass-weighted, the higher-tonnage stream pulls the blend towards its own grade. Setting one stream's grade to 0 models mining dilution by barren waste, which always lowers the delivered head grade below the in-situ ore grade. The formula assumes the metal (grade-tonnes) of each stream simply adds, which holds for grade-by-mass; it errors rather than dividing by zero when total tonnes is zero or a tonnage is negative.
Worked example
Blending 5,000 t of ore with 2,000 t of barren dilution. An open-pit grade-control block delivers 5,000 t of ore at 2.4 g/t gold. During mining it picks up 2,000 t of barren wall-rock dilution at 0 g/t. The blended ROM grade = (5,000 × 2.4 + 2,000 × 0) ÷ (5,000 + 2,000) = 12,000 ÷ 7,000 = 1.7143 g/t, over a total of 7,000 t. Dilution has dropped the delivered head grade from 2.4 g/t to about 1.7143 g/t.
Common mistakes
- Mixing units — entering one grade in g/t and the other in % gives a meaningless blend. Convert both to the same unit first (1 % = 10,000 g/t for a by-mass grade).
- Taking a plain average of the two grades. The ROM grade is weighted by tonnes, so (g1 + g2) ÷ 2 is only correct when the two tonnages are exactly equal.
- Forgetting to include dilution tonnes as a zero-grade stream, which overstates the delivered head grade versus what the mill actually receives.
Frequently asked questions
What units should I use for the grades?
Any consistent grade-by-mass unit works — g/t, ppm (1 ppm = 1 g/t), or % — as long as BOTH streams use the same one. The blended grade comes out in that same unit. Do not mix g/t with %.
How do I model mining dilution?
Enter the ore as one stream (its tonnes and in-situ grade) and the barren dilution as the other stream with a grade of 0. The result is the diluted ROM grade delivered to the crusher, which is always lower than the ore grade.
Can I blend more than two streams?
This tool blends two streams at a time. To combine three or more, blend the first two, then treat the result (its total tonnes and blended grade) as one input stream and blend it with the next.
Is this the same as a reconciled head grade?
It is a theoretical blend estimate from your tonnes and grades. Actual reconciled head grade also reflects survey volumes, moisture, sampling error and mill measurement, so treat this as a planning estimate and verify against site reconciliation.
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