Bolt Torque, Tap Drill & Fastener Grade Chart
Dry tightening torques for metric 8.8/10.9/12.9 and SAE Grade 5/8 bolts (K=0.2, 75% proof), 75% tap drill sizes, and ISO 898-1 / SAE J429 grade strengths.
Updated August 16, 2026
Every torque value below is computed from the short-form torque equation T = K·F·d with the industry-standard chart assumptions: nut factor K = 0.20 for plain, dry steel threads, and preload F equal to 75% of the bolt’s proof load. That is exactly how the published fastener-manufacturer charts (Fastenal technical reference) are built, and our computed cells match them. The important consequence: these torques are only valid dry. Lubricated or zinc-plated threads develop the same preload at 15–25% less torque — using a dry value on a lubricated bolt over-tensions it.
Tap drill sizes are the classic 75%-thread-engagement drills from the Machinery’s Handbook convention: for metric coarse threads the drill is simply the nominal diameter minus the pitch (an M10×1.5 taps at 8.5 mm), rounded to the nearest stocked drill. 75% engagement is the sweet spot — going deeper than that gains under 5% thread strength while tap breakage risk rises sharply.
The grade table gives the specified minimums that the property-class number encodes: an 8.8 bolt has a tensile strength of roughly 800 MPa and yields at about 80% of it (the ".8"). SAE grades mark strength with radial dashes on the head — none for Grade 2, three for Grade 5, six for Grade 8.
Tightening Torque — Metric Coarse, Dry (K = 0.20, 75% Proof Load)
| Size | Pitch (mm) | Class 8.8 (N·m) | Class 10.9 (N·m) | Class 12.9 (N·m) |
|---|---|---|---|---|
| M6 | 1.00 | 10.5 | 15.0 | 17.6 |
| M8 | 1.25 | 25.5 | 36.5 | 42.6 |
| M10 | 1.50 | 50.5 | 72.2 | 84.4 |
| M12 | 1.75 | 88.0 | 126 | 147 |
| M16 | 2.00 | 218 | 312 | 365 |
| M20 | 2.50 | 426 | 610 | 712 |
| M24 | 3.00 | 736 | 1053 | 1231 |
Computed as T = 0.20 × (0.75 × Sp × At) × d with ISO 898-1 proof strengths (580/830/970 MPa) and At = π/4·(d − 0.9382p)². Matches the Fastenal metric chart, which applies 580 MPa to all 8.8 sizes; ISO 898-1 itself lists 600 MPa proof above M16, which would raise those two cells about 3%. Reduce torque 15–25% for lubricated or plated threads.
Tightening Torque — SAE Grade 5 & 8, UNC, Dry (K = 0.20, 75% Proof Load)
| Size (UNC) | Ø (in) | At (in²) | Grade 5 (ft·lb) | Grade 8 (ft·lb) |
|---|---|---|---|---|
| 1/4"-20 | 0.2500 | 0.0318 | 8.4 | 11.9 |
| 5/16"-18 | 0.3125 | 0.0524 | 17.4 | 25 |
| 3/8"-16 | 0.3750 | 0.0775 | 31 | 44 |
| 7/16"-14 | 0.4375 | 0.1063 | 49 | 70 |
| 1/2"-13 | 0.5000 | 0.1419 | 75 | 106 |
| 9/16"-12 | 0.5625 | 0.1819 | 109 | 153 |
| 5/8"-11 | 0.6250 | 0.2260 | 150 | 212 |
| 3/4"-10 | 0.7500 | 0.3345 | 267 | 376 |
| 7/8"-9 | 0.8750 | 0.4617 | 429 | 606 |
| 1"-8 | 1.0000 | 0.6057 | 644 | 909 |
Computed as T = 0.20 × (0.75 × Sp × At) × D with SAE J429 proof strengths (85/120 ksi) and ASME B1.1 tensile stress areas as tabulated in the Fastenal inch chart. Values match that chart within rounding.
Tap Drill Sizes — Metric Coarse (≈75% Thread)
| Thread | Tap drill (mm) | Closest US drill |
|---|---|---|
| M3 × 0.5 | 2.50 | #39 |
| M4 × 0.7 | 3.30 | #30 |
| M5 × 0.8 | 4.20 | #19 |
| M6 × 1.0 | 5.00 | #8 |
| M8 × 1.25 | 6.80 | H |
| M10 × 1.5 | 8.50 | R |
| M12 × 1.75 | 10.30 | 13/32 |
| M16 × 2.0 | 14.00 | 35/64 |
| M20 × 2.5 | 17.50 | 11/16 |
| M24 × 3.0 | 21.00 | — |
Metric rule of thumb: tap drill = nominal diameter − pitch, rounded to the nearest stocked drill (LittleMachineShop 75% column; M24 verified against published metric charts). For hard steels many shops open up toward 65–70% engagement.
Tap Drill Sizes — UNC/UNF (75% Thread)
| Thread | Tap drill | Decimal (in) |
|---|---|---|
| #4-40 UNC | #43 | 0.0890 |
| #6-32 UNC | #36 | 0.1065 |
| #8-32 UNC | #29 | 0.1360 |
| #10-24 UNC | #25 | 0.1495 |
| #10-32 UNF | #21 | 0.1590 |
| #12-24 UNC | #16 | 0.1770 |
| 1/4"-20 UNC | #7 | 0.2010 |
| 1/4"-28 UNF | #3 | 0.2130 |
| 5/16"-18 UNC | F | 0.2570 |
| 5/16"-24 UNF | I | 0.2720 |
| 3/8"-16 UNC | 5/16 | 0.3125 |
| 3/8"-24 UNF | Q | 0.3320 |
| 7/16"-14 UNC | U | 0.3680 |
| 1/2"-13 UNC | 27/64 | 0.4219 |
| 1/2"-20 UNF | 29/64 | 0.4531 |
| 9/16"-12 UNC | 31/64 | 0.4844 |
| 5/8"-11 UNC | 17/32 | 0.5312 |
| 3/4"-10 UNC | 21/32 | 0.6562 |
| 3/4"-16 UNF | 11/16 | 0.6875 |
| 7/8"-9 UNC | 49/64 | 0.7656 |
| 1"-8 UNC | 7/8 | 0.8750 |
| 1"-12 UNF | 15/16 | 0.9375 |
75%-thread column of the LittleMachineShop tap & clearance drill chart (Machinery’s Handbook convention). For steel, stainless, and iron the 50%-thread drill one size up is often preferred.
Fastener Grades — Specified Minimum Strengths & Head Markings
| Grade / class | Size range | Proof | Yield (min) | Tensile (min) | Head marking |
|---|---|---|---|---|---|
| ISO class 8.8 (≤ M16) | M5–M16 | 580 MPa | 640 MPa | 800 MPa | "8.8" numerals |
| ISO class 8.8 (> M16) | M18–M39 | 600 MPa | 660 MPa | 830 MPa | "8.8" numerals |
| ISO class 10.9 | M5–M39 | 830 MPa | 940 MPa | 1040 MPa | "10.9" numerals |
| ISO class 12.9 | M1.6–M39 | 970 MPa | 1100 MPa | 1220 MPa | "12.9" numerals |
| SAE J429 Grade 2 | 1/4"–3/4" | 55 ksi | 57 ksi | 74 ksi | No marks |
| SAE J429 Grade 5 | 1/4"–1" | 85 ksi | 92 ksi | 120 ksi | 3 radial lines |
| SAE J429 Grade 8 | 1/4"–1" | 120 ksi | 130 ksi | 150 ksi | 6 radial lines |
ISO 898-1 property classes read as tensile/10 and yield ratio: 10.9 → ~1000 MPa tensile, yield ≈ 0.9 × tensile. SAE Grade 2 values shown apply through 3/4 in (they drop above); Grade 5 above 1 in drops to 74 ksi proof. Proof loads, not yield, anchor preload specs.
Sources & Further Reading
- Fastenal Technical Reference — Torque-Tension Relationship for Metric Fasteners (Rev 3-4-09) and for A307A, Grade 5, 8 & 9 Bolts (Rev 3-6-09): K = 0.20 dry, clamp = 75% proof load; torque cells computed and matched
- ISO 898-1 — Mechanical properties of fasteners: property-class proof/yield/tensile minimums
- SAE J429 — Mechanical and material requirements for externally threaded fasteners (Portland Bolt / STS Industrial reproductions)
- LittleMachineShop.com Tap & Clearance Drill Sizes chart (Machinery’s Handbook 75%-thread convention); M24 drill verified across published metric charts
Frequently Asked Questions
Why must I reduce these torques for lubricated bolts?
Torque is mostly spent on friction — only about 10% becomes bolt stretch. Lubrication drops the nut factor from ~0.20 to ~0.15, so the same torque produces roughly a third more preload, enough to yield the bolt. Use the K that matches the actual thread condition: ~0.20 dry, ~0.17 zinc-plated, ~0.15 lubricated.
What does the 8.8 on a metric bolt head actually mean?
First digit: nominal tensile strength in hundreds of MPa (8 → 800 MPa). Second digit: yield as a tenth-fraction of tensile (.8 → yield ≈ 640 MPa). So 12.9 decodes as 1200 MPa tensile with yield at 90% of that. Proof stress — what torque charts are built on — sits slightly below yield.
Is a 75% thread tap drill always the right choice?
It is the default for aluminum, brass, and mild steel. In tough materials (stainless, tool steel) most references drop to 50–65% engagement — the joint loses almost no strength because threads strip at the root regardless, but the tap survives. That is why charts publish separate 75% and 50% columns.
Are SAE Grade 5 and metric class 8.8 interchangeable?
They are close cousins — Grade 5 proof is 85 ksi (~586 MPa) versus 580 MPa for 8.8 up to M16 — but the threads are incompatible and the head markings differ. Match the fastener system to the equipment; never force a near-size metric/inch substitution.
Try the Calculators
Bolt Torque, Tap Drill & Fastener Grade Chart — reuven.tools/reference/bolt-torque-tap-drill — verified against: Fastenal Technical Reference — Torque-Tension Relationship for Metric Fasteners (Rev 3-4-09) and for A307A, Grade 5, 8 & 9 Bolts (Rev 3-6-09): K = 0.20 dry, clamp = 75% proof load; torque cells computed and matched; ISO 898-1 — Mechanical properties of fasteners: property-class proof/yield/tensile minimums; SAE J429 — Mechanical and material requirements for externally threaded fasteners (Portland Bolt / STS Industrial reproductions); LittleMachineShop.com Tap & Clearance Drill Sizes chart (Machinery’s Handbook 75%-thread convention); M24 drill verified across published metric charts