# Flange Bolt Torque Calculator (Target Preload and Nut Factor)

> Source: https://netray.co/tools/flange-bolt-torque-calculator-from-preload-and-nut-factor
> Publisher: Netray (netray.co), AI and integration engineering for ERP and manufacturing systems.
> Section: Free Tools

## Short answer

Compute bolt stress area, target preload and tightening torque from bolt size, pitch, yield strength, percent of yield and nut factor.

## Category

maintenance

## Industry

Oil and Gas Equipment

## Audience

- Maintenance technician
- Piping engineer
- Field service engineer
- Mechanical engineer

## Tool

**Kind:** calculator

**Inputs:**

#### Nominal bolt diameter

**Key:** d

**Type:** number

**Default:** 24

**Min:** 6

**Step:** 1

**Unit:** mm

**Help:** M24 is about 7/8 in.

#### Thread pitch

**Key:** p

**Type:** number

**Default:** 3

**Min:** 0.5

**Step:** 0.25

**Unit:** mm

**Help:** M24 coarse pitch is 3 mm. Large studs often use 8 threads per inch, 3.175 mm.

#### Bolt material yield strength

**Key:** ys

**Type:** number

**Default:** 720

**Min:** 100

**Step:** 10

**Unit:** MPa

**Help:** ASTM A193 B7 up to 2.5 in is 105 ksi, about 720 MPa.

#### Target preload, percent of yield

**Key:** pct

**Type:** slider

**Default:** 50

**Min:** 30

**Max:** 75

**Step:** 1

**Unit:** %

**Help:** Many flange procedures use about 50 percent. Follow your own procedure.

#### Nut factor K

**Key:** k

**Type:** select

**Default:** 0.2

**Options:**

##### Dry or plain steel, K 0.20

**Value:** 0.2

##### Oil lightly lubricated, K 0.16

**Value:** 0.16

##### Moly paste or nickel anti-seize, K 0.13

**Value:** 0.13

##### PTFE coated, K 0.12

**Value:** 0.12

#### Number of bolts

**Key:** n

**Type:** number

**Default:** 12

**Min:** 1

**Step:** 1

**Unit:** pcs

**Outputs:**

#### Tensile stress area

**Key:** as

**Expr:** pi / 4 * pow(d - 0.9382 * p, 2)

**Format:** decimal

**Description:** ISO metric stress area in mm2.

#### Target preload per bolt

**Key:** f

**Expr:** as * ys * pct / 100 / 1000

**Format:** decimal

**Description:** In kN.

#### Torque per bolt

**Key:** t

**Expr:** k * d * f

**Format:** decimal

**Primary:** true

**Description:** T = K x d x F, result in N.m with d in mm and F in kN.

#### Torque per bolt

**Key:** tft

**Expr:** t * 0.737562

**Format:** decimal

**Primary:** true

**Description:** In ft-lb.

#### Total preload, all bolts

**Key:** tot

**Expr:** f * n

**Format:** decimal

**Description:** In kN, before elastic interaction and relaxation.

#### Bolt stress at target

**Key:** stress

**Expr:** ys * pct / 100

**Format:** decimal

**Description:** In MPa.

**Chart:**

#### Torque per bolt vs target percent of yield

**Type:** line

**X:** pct

**From:** 30

**To:** 75

**Steps:** 15

**Series:**

- t

**Disclaimer:** Torque-preload scatter is large, commonly plus or minus 25 percent or more with friction uncertainty. Use the flange manufacturer or gasket supplier procedure for critical joints, and tension or hydraulic tools where required.

## Content

**Intro:** The short form torque equation T = K x d x F converts a target bolt load into a wrench setting. This calculator derives the load from bolt size and strength, so you can work with any stud, not just a table value.

**Sections:**

#### How to use the calculator

**Body:** Enter nominal diameter and pitch to get the stress area. Enter the bolt yield strength and the percent of yield you want to reach, then choose the friction condition through the nut factor. The result is torque per bolt in N.m and ft-lb, plus the total clamp load across the joint. The sweep chart shows how sensitive torque is to the percent of yield chosen.

**Bullets:**

- Use the lubricant actually applied to threads and nut faces
- Use the same value across the whole pattern
- Re-check with the gasket manufacturer's stress target
- Torque in stages and in a star pattern

#### The equations

**Body:** Tensile stress area for ISO metric threads is pi/4 x (d minus 0.9382 x P) squared. Preload is stress area times yield times the target fraction. Torque is nut factor K times nominal diameter times preload. Typical K ranges from about 0.12 for coated or heavily lubricated threads to about 0.2 or more for dry steel. A small change in K moves torque by the same proportion, which is why lubrication must be controlled.

#### Worked example

**Body:** M24 x 3 B7 stud, yield 720 MPa, 50 percent target, K 0.2. Stress area is about 353 mm2. Preload is 353 x 720 x 0.5 divided by 1000, about 127 kN. Torque equals 0.2 x 24 x 127, about 610 N.m or 450 ft-lb. With moly paste at K 0.13 the same preload needs only about 400 N.m. Using the dry value on a lubricated stud would overload it badly.

#### Common mistakes and ERP notes

**Body:** Frequent errors are using the dry torque with lubricated studs, skipping the star pattern, tightening in one pass, and ignoring that gasket type sets the required stress. Maintenance modules in SAP PM or Infor EAM can hold the torque value on a task list so technicians do not rely on memory.

**Cta:** To get torque values and procedures onto your work orders automatically, Netray can discuss options with your ERP or maintenance team. The calculator is free to use.

**Faq:**

#### What is the nut factor K?

**Answer:** It is a lumped friction coefficient in T = K x d x F. Dry steel is around 0.2, lubricated threads around 0.13 to 0.16. Real values vary, so use test data for critical joints.

#### What percent of yield should I use for flange studs?

**Answer:** Many procedures target roughly 40 to 60 percent of yield, depending on gasket and joint design. Use your flange or gasket supplier's recommended bolt stress or your site procedure.

#### Is torque accurate for preload?

**Answer:** Not very. Friction scatter can leave actual preload off by 25 percent or more. For critical joints use hydraulic tensioning or controlled bolting with calibrated tools.

#### Does this work for inch bolts?

**Answer:** Yes. Convert diameter and pitch to mm and strength to MPa. For UNC 1 inch 8 threads per inch bolts, diameter is 25.4 mm and pitch 3.175 mm.

## Search terms

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