What is Rock Mass Rating (RMR)?

Rock Mass Rating (RMR) is a geomechanics classification system developed by Bieniawski (1973, revised 1989). It rates a rock mass on six parameters — UCS, RQD, discontinuity spacing, discontinuity condition, groundwater, and orientation adjustment — producing a score from 0 to 100. It is the most widely used rock classification system in tunneling, mining, and slope engineering worldwide.

What is a good RMR score?

Scores of 81–100 (Class I) indicate very good rock. Scores of 61–80 (Class II) are good rock. Scores of 41–60 (Class III) are fair and require systematic support. Scores below 40 (Classes IV and V) represent poor to very poor rock requiring heavy, immediate support.

What is the difference between RMR76 and RMR89?

RMR89 (Bieniawski 1989) is the current industry standard. It has revised ratings for UCS and RQD, expands discontinuity condition into five sub-parameters, and updates the groundwater rating scale. Always use the 1989 version unless a project specifically requires the earlier edition.

How does RMR relate to the Q-System?

Both are empirical rock classification tools. The common correlation is: RMR ≈ 9 × ln(Q) + 44. On important projects, calculate both independently and compare. Significant divergence signals unusual site conditions worth investigating further.

How do I calculate RQD without core drilling?

Use Palmstrom's formula: RQD = 115 − 3.3 × Jv, where Jv is the number of joints per cubic meter. Alternatively use Priest and Hudson (1976): RQD = 100 × e^(−0.1λ) × (0.1λ + 1), where λ is mean joint frequency per meter.

Is this RMR calculator suitable for professional reports?

This calculator correctly implements Bieniawski 1989 and supports professional work as a computation aid. All input parameters must be assessed by a qualified geotechnical or mining engineer from actual field investigations and lab tests before use in final design.

📖 Complete Guide

How to Calculate Rock Mass Rating (RMR)

🗓 Updated April 2026 ⏱ 8 min read 📐 Bieniawski 1989

Rock Mass Rating (RMR) is the most widely used geomechanics classification system in the United States. This complete step-by-step guide covers every parameter, all rating tables, and a worked example — so you can calculate RMR confidently for any tunneling, mining, or slope project.

The RMR Formula

Rock Mass Rating is calculated by summing the ratings of six parameters. Each parameter is rated from field data or lab tests and assigned a numeric score:

Bieniawski 1989 — RMR Formula

RMR = R1 + R2 + R3 + R4 + R5 + R6

Where:
R1 = Uniaxial Compressive Strength rating (0–15)
R2 = RQD rating (3–20)
R3 = Spacing of discontinuities rating (5–20)
R4 = Condition of discontinuities rating (0–30)
R5 = Groundwater rating (0–15)
R6 = Orientation adjustment (0 to −12 for tunnels)

Score range: RMR runs from 0 (worst) to 100 (best). A higher score means better rock quality, longer stand-up time, and less support needed. The maximum possible Basic RMR (before orientation) is 100 points.

Step-by-Step Calculation

Rate Uniaxial Compressive Strength (UCS)

Run a lab UCS test, point load test, or Schmidt hammer. Match your result to the table below and record the rating (R1).

Max: 15 pts

Calculate RQD and Get Rating

From core logs: add length of all pieces ≥ 100 mm (4 in) ÷ total core run × 100. Enter the percentage in the table below for rating (R2).

Max: 20 pts

Measure Discontinuity Spacing

Measure average distance between the most dominant joint set or bedding planes. Match to table for rating (R3).

Max: 20 pts

Assess Discontinuity Condition (5 sub-parameters)

Rate persistence, aperture, roughness, infilling, and weathering separately. Sum all five — the total is capped at 30 for rating (R4).

Max: 30 pts

Evaluate Groundwater Conditions

Observe water inflow at the tunnel face, borehole, or slope. Rate conservatively — conditions change seasonally. Record rating (R5).

Max: 15 pts

Apply Orientation Adjustment

Evaluate how the dominant joint set's strike and dip relates to your excavation direction. Apply the penalty (R6) — this is the only negative parameter.

0 to −12

Table 1 — Uniaxial Compressive Strength Rating

Determine UCS from a lab test, point load index (UCS ≈ 24 × Is50), or Schmidt hammer. Select the closest range:

UCS (MPa)	UCS (psi)	Rock Type Examples	Description	Rating (R1)
> 250 MPa	> 36,000 psi	Fresh granite, quartzite, basalt	Very strong	15
100–250 MPa	14,500–36,000 psi	Limestone, dolerite, hard sandstone	Strong	12
50–100 MPa	7,250–14,500 psi	Sandstone, slate, moderately fresh rock	Medium strong	7
25–50 MPa	3,625–7,250 psi	Weathered granite, soft limestone	Moderate	4
5–25 MPa	725–3,625 psi	Coal, chalk, highly weathered rock	Weak	2
1–5 MPa	145–725 psi	Soft shale, weak mudstone	Very weak	1
< 1 MPa	< 145 psi	Plastic clay, decomposed rock	Extremely weak	0

Table 2 — Rock Quality Designation (RQD) Rating

RQD is calculated from core samples. If core drilling is not available, use Palmstrom's formula: RQD = 115 − 3.3 × Jv where Jv = number of joints per cubic meter.

RQD (%)	Description	Rating (R2)
90–100%	Excellent	20
75–90%	Good	17
50–75%	Fair	13
25–50%	Poor	8
< 25%	Very poor	3

Table 3 — Spacing of Discontinuities Rating

Measure the mean spacing between the most dominant discontinuity set (joints, bedding, faults). Use the closest range:

Spacing (m)	Spacing (ft / in)	Description	Rating (R3)
> 2 m	> 6.5 ft	Very wide	20
0.6–2 m	2–6.5 ft	Wide	15
0.2–0.6 m	8 in – 2 ft	Moderate	10
0.06–0.2 m	2.4–8 in	Close	8
< 0.06 m	< 2.4 in	Very close	5

Table 4 — Condition of Discontinuities (5 Sub-parameters)

Rate each sub-parameter separately and sum them. The total is automatically capped at 30 points (R4 max = 30).

4a. Persistence (Joint Length)

Persistence	Rating
< 1 m (< 3.3 ft)	6
1–3 m (3.3–10 ft)	4
3–10 m (10–33 ft)	2
10–20 m (33–66 ft)	1
> 20 m (> 66 ft)	0

4b. Aperture (Joint Opening)

Aperture	Rating
None (closed / healed)	6
< 0.1 mm (< 0.004 in)	5
0.1–1 mm (0.004–0.04 in)	4
1–5 mm (0.04–0.2 in)	1
> 5 mm (> 0.2 in)	0

4c. Roughness

Surface Roughness	Rating
Very rough (stepped / undulating)	6
Rough	5
Slightly rough	3
Smooth	1
Slickensided (polished striations)	0

4d. Infilling (Gouge Material)

Infill Type	Rating
None — rock wall contact	6
Hard infill < 5 mm (calcite, quartz)	4
Hard infill > 5 mm	2
Soft infill < 5 mm (clay, fault gouge)	2
Soft infill > 5 mm	0

4e. Weathering of Joint Walls

Weathering Grade	Description	Rating
Unweathered (W1)	No visible alteration, fresh rock	6
Slightly weathered (W2)	Minor surface staining, strength unchanged	5
Moderately weathered (W3)	Partial decomposition, reduced strength	3
Highly weathered (W4)	Significant decomposition, friable	1
Completely decomposed (W5)	Soil-like material, original texture lost	0

Table 5 — Groundwater Condition Rating

Observe actual conditions at the tunnel face, borehole, or slope. When conditions are uncertain, always rate conservatively (lower score).

Condition	Inflow per 10 m tunnel	Description	Rating (R5)
Completely dry	None	No moisture observed	15
Damp	< 10 L/min	Moisture but no free water	10
Wet	10–25 L/min	Water trickling from joints	7
Dripping	25–125 L/min	Steady drip from crown	4
Flowing	> 125 L/min	High inflow, may cause instability	0

Table 6 — Discontinuity Orientation Adjustment

This is the only parameter that reduces the score. The penalty depends on both the favorability of the joint orientation AND the type of structure being assessed:

Favorability	Tunnels & Mines	Foundations	Slopes
Very Favorable — joints dip into excavation	0	0	0
Favorable	−2	−2	−5
Fair	−5	−7	−25
Unfavorable	−10	−15	−50
Very Unfavorable — joints dip out of slope	−12	−25	−60

⚠ Note for slopes: Slope orientation penalties are much larger (up to −60) because adverse joint dip direction is the primary failure mechanism in slope instability. Always use the correct application type when calculating RMR.

Rock Mass Rating Calculation — Classification Table

Once you have summed all six parameters, use this table to determine the rock class and engineering properties:

RMR Score	Class	Rock Quality	Stand-up Time	Cohesion	Friction Angle
81–100	Class I	Very Good Rock	20 years / 15 m span	> 400 kPa (> 58 psi)	> 45°
61–80	Class II	Good Rock	1 year / 10 m span	300–400 kPa (43–58 psi)	35–45°
41–60	Class III	Fair Rock	1 week / 5 m span	200–300 kPa (29–43 psi)	25–35°
21–40	Class IV	Poor Rock	10 hours / 2.5 m span	100–200 kPa (15–29 psi)	15–25°
0–20	Class V	Very Poor Rock	30 minutes / 1 m span	< 100 kPa (< 15 psi)	< 15°

Worked Example — Sandstone Tunnel

Let's calculate RMR for a highway tunnel in medium-strong sandstone with fair groundwater conditions. All field data collected from core logging and site investigation:

Site: Highway Tunnel — Sandstone Formation, Colorado

Application: Tunnel / Mine · Bieniawski 1989

R1 — UCS: 65 MPa (medium strong sandstone)50–100 MPa range7 pts

R2 — RQD: 72% (good core recovery)50–75% range13 pts

R3 — Spacing: 0.4 m between joints0.2–0.6 m range10 pts

R4 — Discontinuity Condition:sub-total below—

4a. Persistence: 3–10 m+2

4b. Aperture: 0.1–1 mm+4

4c. Roughness: Slightly rough+3

4d. Infilling: None+6

4e. Weathering: Slightly weathered+520 pts

R5 — Groundwater: Wet (10–25 L/min)Wet7 pts

R6 — Orientation: Favorable (joints dip into tunnel)Favorable−2 pts

Total RMR = 7 + 13 + 10 + 20 + 7 − 2 55

✅ Result: Class III — Fair Rock | Stand-up time: 1 week for 5 m span | Cohesion: 200–300 kPa | Friction: 25–35°
Recommended support: Systematic 4 m rock bolts at 1.5–2 m spacing + 50–100 mm shotcrete in crown and walls.

Common Mistakes to Avoid

Averaging RQD across different rock types — always calculate RMR separately for each geotechnical unit. Different lithologies must not be mixed.
Skipping the orientation adjustment — many engineers ignore R6 but it can subtract up to 12 points for tunnels and 60 points for slopes.
Rating groundwater too optimistically — use the worst expected conditions, not dry-season drilling conditions. Water significantly reduces rock mass strength.
Using one RMR value for the whole project — run a sensitivity analysis on uncertain parameters. Report a range, not a single number.
Misidentifying the dominant joint set — the most continuous, closest-spaced joint set controls behavior. Rating the wrong set produces unsafe results.

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Skip the manual tables. Our free calculator does all six parameters in real time with a downloadable PDF report.

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