Use this bolt length calculator to estimate the required bolt length for a through bolted joint with a nut or for a bolt screwed into a tapped hole. The calculator adds the grip thickness, washer thickness, nut height, thread protrusion or thread engagement length to determine the required bolt length.
This calculator estimates the required bolt length for common bolted joint arrangements. It can calculate the length for a through bolt with a nut, or for a bolt screwed into a tapped hole.
The calculator provides:
This calculator is useful for preliminary fastener selection, assembly planning and checking whether a selected bolt length is long enough for the joint stack.
Through bolt with nut:
Bolt into tapped hole:
Length margin:
Length utilisation:
| Symbol | Description | Units |
|---|---|---|
| L | Required bolt length | mm |
| G | Grip thickness / clamped material thickness | mm |
| W | Total washer thickness | mm |
| N | Nut height | mm |
| P | Required thread protrusion beyond nut | mm |
| Le | Required thread engagement length | mm |
| Lselected | Selected available bolt length | mm |
| Joint Type | Length Includes |
|---|---|
| Through bolt with nut | Grip thickness + washer thickness + nut height + thread protrusion |
| Bolt into tapped hole | Grip thickness + washer thickness + required thread engagement length |
Enter the joint dimensions, then click calculate.
| Result | Meaning |
|---|---|
| Required bolt length | The estimated minimum bolt length needed for the entered joint stack. |
| Selected bolt length | The available or proposed bolt length entered by the user. |
| Length margin | The selected length minus the required length. A positive value means the selected bolt is long enough. |
| Length utilisation | The required length divided by the selected length. A value below 100% means the selected bolt length is sufficient. |
| Pass/fail result | Pass means the selected bolt length is at least equal to the calculated required length. |
This calculator gives a simple bolt length estimate. It does not replace checking the actual fastener standard, thread length or assembly drawing.
| This Calculator Includes | This Calculator Does Not Include |
|---|---|
| Through bolt length estimate | Standard bolt length availability |
| Tapped hole bolt length estimate | Thread runout allowance |
| Washer, nut and protrusion allowance | Blind hole bottom clearance |
| Selected length comparison | Partial thread length checks |
| Worked calculation | Counterbores, recesses or special head geometry |
For calculating required thread engagement length, use the Thread Engagement Length Calculator.
For estimating tightening torque from target preload, use the Bolt Tightening Torque Calculator.
For checking bolt tensile stress, use the Bolt Tensile Stress Calculator.
Bolt length is usually measured from under the bolt head to the end of the bolt for standard hex bolts and cap screws. The head height is normally not included in the nominal bolt length.
For a through bolted joint, the length must pass through the clamped parts, washers and nut, with sufficient thread protrusion beyond the nut. For a tapped hole, the length must pass through the clamped material and provide enough thread engagement in the tapped hole.
For blind tapped holes, the selected bolt length must also leave suitable bottom clearance so the bolt does not bottom out before clamping the joint.
For further fastener data, see the RoyMech reference pages: Bright Hexagon Screws, Nuts and Washers and Bolt Head Clearances.
For the full calculator section, see: Bolted Joint Calculators.
For most standard bolts and cap screws, bolt length is measured from under the head to the end of the bolt. The head height is not normally included in the nominal bolt length.
For a simple through bolt with nut, add the grip thickness, washer thickness, nut height and required thread protrusion.
For a tapped hole, add the grip thickness, washer thickness and required thread engagement length. For blind holes, also check that the bolt does not bottom out.
A common practical approach is to allow at least one or two threads beyond the nut, but the required protrusion depends on the application, standard and inspection requirement.
No. This calculator estimates bolt length. To check thread engagement strength, use the Thread Engagement Length Calculator.
This calculator is intended for preliminary engineering calculations and assembly planning. Final fastener selection should check standard bolt lengths, thread length, joint drawing requirements and any applicable design standard.