Flange Bolt Size Chart + Guide

While it’s tempting to use whatever you have in hand, improper use of flange bolt sizes can cause an array of potentially dangerous accidents and damages.  

To avoid such risks, it’s imperative to visit a comprehensive flange bolt size chart to make sure you’re using the right size for your projects.

This one-stop bolt size and length chart helps you with just that.

In this article, I’ll show you how to properly read a flange bolt size chart, as well as identify what size flange bolt you already have. Let’s dive in! 

How to Read a Flange Bolt Size Chart

Reading a flange bolt size chart is fairly straightforward. 

The size guide below lists accurate sizing information about the selected flange’s diameter and length, as well as the required wrench size and torque patterns. 

Here’s how to read a flange bolt size chart: 

Select the Right NPS 

Nominal Pipe Size, abbreviated to NPS, is a set of standard North American sizes for pipes used in high or low temperatures and pressures. 

To be clear, the NPS listed below refers to the approximate outside diameter (OD) of a pipe rather than its exact figure. 

This means that you have to round up your pipe measurement to get the NPS required. 

For instance, if your pipe’s OD measures 2.375 inches (60.3 mm), your NPS is 2. 

Flange bolts come in the following flange sizes: 

• 150 lb.
• 300 lb.
• 600 lb.
• 900 lb.
• 1500 lb.
• 2500 lb.

Count the Number of Bolts Required

The next set of columns corresponds with the size of the pipe’s NPS. 

The number of bolts refers to the actual number of bolts required according to the NPS. Naturally, the larger the pipe, the more bolts it requires. 

For your reference, these are the corresponding number of bolts based on the NPS: 

• 1/4-inch to 3-inch NPS: 4 bolts
• 3-1/2-inches to 8-inch NPS: 8 bolts
• 10-inch to 14-inch NPS: 12 bolts
• 16-inch to 18-inch NPS: 16 bolts 
• 20-inch to 24-inch NPS: 20 bolts 

Determine the Flange Bolt Diameter  

The next set of columns represent the flange bolt diameter and length. 

The diameter lists the size of the bolt’s holes, circle, and flange, all of which are necessary when selecting the right flange bolt for your project. 

Bolt holes come in six primary sizes: 5/8-inch, 3/4-inch, 7/8-inch, 1-inch, 1 1/8-inch, and 1 1/4-inch. 

5/8-inch bolt holes fit 1/4 to 1-1/2-inch NPS, 3/4-inch bolt holes fit 2 to 3-inch NPS, and so on. 

The bolt circle and flange come with their own unique measurements, ranging from 2-1/4 to 29-1/2, and 3-3/8-inch to 32-inch respectively. 

Determine the Flange Bolt Length 

Once you’ve determined the diameter of the flange bolt, you’re ready to jump to the next column: bolt length. 

Bolt length is divided into two segments: Raised Face Flange (RF) and Ring Type Joint Flange (RTJ). 

Raised Face Flanges are typically used in process plant applications.

They’re fairly easy to identify because they’re the only type of flange with their gasket surface situated above the bolting circle face, giving them a “raised” appearance. 

RF flange diameters and heights belong in the ASME B16.5 group, which are limited to flanged fittings made from forged or cast materials. 

On the other hand, Ring Type Joint flanges are used in high pressure and/or high-temperature services above 800°F, as commonly seen in Petroleum and Petrochemical industries.

For this reason, they typically have a Class 600 and higher rating. 

RTJ flanges have deep, high tolerance grooves cut into their faces. The grooves rest on metal rings that compress whenever the connecting bolts are tightened. 

RTJ flanges are manufactured to ASME B16.20 Standards. They’re best used with flanges made to API 6A, ANSI B16.5, and ASME B16.47 Series A Standards.

Wrench Size and Suggested Torque Measurements 

The final two columns list the recommended wrench and torque size according to the flange bolt diameter and length. 

Just like with any bolt size chart, the wrench and torque size determines the amount of force you can use to tighten the bolt. 

Always select the correct wrench and torque size to prevent injuries and damage to equipment due to slippage or the application of too much pressure. 

Real-Life Flange Bolt Size Chart Application 

Let’s say you need to bolt a flange to a pipe. 

First, measure the pipe’s NPS. If the pipe measures 3 5/8 inches, check the measurements under the 3 NPS classification and gather the materials according to the listed information: 

NPS	No. of Bolts	Bolt Diameter			Bolt Length		Wrench Size	Recommended Torque (Ft./Lbs.)
		Bolt Holes	Bolt Circle	Flange	Raised Face	Ring Joint
3	4	3/4	6	7-1/2	3 1/2	4	1-1/16	110

With the chart as a guide, select the proper flange bolt size, quantity, diameter, and length for the equipment being assembled. 

Insert all four bolts and tighten all the nuts by hand until snug. 

As listed in the chart, use a size 1-1/16 wrench and tighten the flange bolt in 5 ft-lb increments until the final torque value—110—is reached. 

Over-torquing can damage the flange so make sure to stop once the recommended torque is reached.

Standard Flange Bolt Size Chart

Class 150 Flanges

NPS	No. of Bolts	Bolt Diameter			Bolt Length		Wrench Size	Recommended Torque (Ft./Lbs.)
		Bolt Holes	Bolt Circle	Flange	Raised Face	Ring Joint
1/4	4	5/8	2-1/4	3-3/8	2 1/4	–	–	–
1/2	4	5/8	2-3/8	3-1/2	2 1/2	–	7/8	30
3/4	4	5/8	2-3/4	3-7/8	2 1/2	3	7/8	30
1	4	5/8	3-1/8	4-1/4	2 3/4	3 1/4	7/8	40
1-1/4	4	5/8	3-1/2	4-5/8	2 3/4	3 1/4	7/8	40
1-1/2	4	5/8	3-7/8	5	3 1/4	3 3/4	7/8	40
2	4	3/4	4-3/4	6	3 1/2	4	1-1/16	80
2-1/2	4	3/4	5-1/2	7	3 1/2	4	1-1/16	80
3	4	3/4	6	7-1/2	3 1/2	4	1-1/16	110
3-1/2	8	3/4	7	8-1/2	3 1/2	4	1-1/16	80
4	8	3/4	7-1/2	9	3 3/4	4 1/4	1-1/16	100
5	8	7/8	8-1/2	10	4	4 1/2	1-1/4	120
6	8	7/8	9-1/2	11	4 1/4	4 3/4	1-1/4	130
8	8	7/8	11-3/4	13-1/2	4 1/2	5	1-1/4	130
10	12	1	14-1/4	16	4 3/4	5 1/4	1-7/16	215
12	12	1	17	19	5 1/4	5 3/4	1-7/16	220
14	12	1 1/8	18-3/4	21	5 1/4	5 3/4	1-5/8	320
16	16	1 1/8	21-1/4	23-1/2	5 3/4	6 1/4	1-5/8	320
18	16	1 1/4	22-3/4	25	6 1/4	6 3/4	1-13/16	450
20	20	1 1/4	25	27-1/2	6 3/4	7 1/4	1-13/16	450
24	20	1 1/4	29-1/2	32	7	7-3/4	2	650

 

Class 300 Flanges

NPS	No. of Bolts	Bolt Diameter			Bolt Length		Wrench Size	Recommended Torque (Ft./Lbs.)
		Bolt Holes	Bolt Circle	Flange	Raised Face	Ring Joint
1/4	4	12/30	2-1/4	3-3/8	2-1/2	3	–	–
1/2	4	12/30	2-5/8	3-3/4	3	3-1/4	7/8	40
3/4	4	12/30	3-1/4	4-5/8	3-1/4	3-1/4	1-1/16	60
1	4	12/30	3-1/2	4-7/8	3-1/4	3-1/4	1-1/16	80
1-1/4	4	12/30	3-7/8	5-1/4	3-1/2	4	1-1/16	80
1-1/2	4	12/30	4-1/2	6-1/8	3-1/2	4-1/4	1-1/4	110
2	8	12/30	5	6-1/2	4	4-1/2	1-1/16	110
2-1/2	8	12/30	5-7/8	7-1/2	4-1/4	5	1-1/4	150
3	8	12/30	6-5/8	8-1/4	4-1/4	5	1-1/4	150
3-1/2	8	12/30	7-1/4	9	4-1/2	5-1/4	1-1/4	150
4	8	12/30	7-7/8	10	4-3/4	5-1/4	1-1/4	180
5	8	12/30	9-1/4	11	4-3/4	5-3/4	1-1/4	180
6	12	12/30	10-5/8	12-1/2	5-1/2	6-1/4	1-1/4	170
8	12	12/31	13	15	6-1/4	7-1/4	1-7/16	265
10	16	1.12	15-1/4	17-1/2	6-3/4	7-1/2	1-5/8	320
12	16	1.25	17-3/4	20-1/2	7	7-3/4	1-13/16	450
14	20	1.25	20-1/4	23	7-1/2	8-1/4	1-13/16	450
16	20	1.38	22-1/2	25-1/2	7-3/4	8-1/2	2	650
18	24	1.38	24-3/4	28	8-1/4	9-1/4	2	650
20	24	1.38	27	30-1/2	8-3/4	9-3/4	2	650
24	24	1.62	32	36	9-1/4	10-1/4	2-3/8	1200

 

Class 600 Flanges

NPS	No. of Bolts	Bolt Diameter			Bolt Length		Wrench Size	Recommended Torque (Ft./Lbs.)
		Bolt Holes	Bolt Circle	Flange	Raised Face	Ring Joint
1/2	4	0.62	2-5/8	3-3/4	3	3	7/8	40
3/4	4	0.75	3-1/4	4-5/8	3-1/4	3-1/4	1-1/16	60
1	4	0.75	3-1/2	4-7/8	3-1/2	3-1/2	1-1/16	80
1-1/4	4	0.75	3-7/8	5-1/4	3-3/4	3-3/4	1-1/16	110
1-1/2	4	0.88	4-1/2	6-1/8	4-1/4	4-1/4	1-1/4	110
2	8	0.75	5	6-1/2	4-1/4	4-1/2	1-1/16	110
2-1/2	8	0.88	5-7/8	7-1/2	5	5-1/4	1-1/4	150
3	8	0.88	6-5/8	8-1/4	5	5-1/4	1-1/4	150
3-1/2	8	1	7-1/4	9	5-1/2	5-1/2	1-7/16	200
4	8	1	8-1/2	10-3/4	5-3/4	6	1-7/16	225
5	8	1.12	10-1/2	13	6-1/2	6-1/2	1-5/8	350
6	12	1.12	11-1/2	14	6-3/4	7	1-5/8	320
8	12	1.25	13-3/4	16-1/2	7-3/4	8	1-13/16	450
10	16	1.38	17	20	8-1/2	8-3/4	2	650
12	20	1.38	19-1/4	22	8-3/4	9	2	675
14	20	1.5	20-3/4	23-3/4	9-1/4	9-1/2	2-5/8	820
16	20	1.62	23-3/4	27	10	10-1/4	2-3/8	1125
18	20	1.75	25-3/4	29-1/4	10-3/4	11	2-9/16	1430
20	24	1.75	28-1/2	32	11-1/2	11-3/4	2-9/16	1400
24	24	2	33	37	13	13-1/2	2-15/16	2230

 

Class 900 Flanges

NPS	No. of Bolts	Bolt Diameter			Bolt Length		Wrench Size	Recommended Torque (Ft./Lbs.)
		Bolt Holes	Bolt Circle	Flange	Raised Face	Ring Joint
1/2	4	0.88	3-1/4	4-3/4	4	4-1/4	1-1/4	80
3/4	4	0.88	3-1/2	5-1/8	4-1/4	4-1/2	1-1/4	90
1	4	1	4	5-7/8	4-3/4	5	1-7/16	110
1-1/4	4	1	4-3/8	6-1/4	5	5	1-7/16	170
1-1/2	4	1.12	4-7/8	7	5-1/4	5-1/2	1-5/8	240
2	8	1	6-1/2	8-1/2	5-1/2	5-3/4	1-7/16	170
2-1/2	8	1.12	7-1/2	9-5/8	6-1/4	6-1/4	1-5/8	240
3	8	1	7-1/2	9-1/2	5-1/2	5-3/4	1-7/16	215
4	8	1.25	9-1/4	11-1/2	6-1/2	6-3/4	1-13/16	450
5	8	1.38	11	13-3/4	7-1/2	7-1/2	2	650
6	12	1.25	12-1/2	15	7-1/2	7-3/4	1-13/16	455
8	12	1.5	15-1/2	18-1/2	8-1/2	8-3/4	1-3/16	820
10	16	1.5	18-1/2	21-1/2	9	9-1/4	2-3/16	850
12	20	1.5	21	24	9-3/4	10	2-3/16	870
14	20	1.62	22	25-1/4	10-1/2	11	2-5/8	1125
16	20	1.75	24-1/2	27-3/4	11	11-1/2	2-9/16	1430
18	20	2	27	31	12-3/4	13-1/4	2-15/16	2230
20	20	2.12	29-1/2	33-3/4	13-1/2	14-1/4	3-1/8	2300
24	20	2.62	35-1/2	41	17	18	3-7/8	5500

 

Class 1500 Flanges

NPS	No. of Bolts	Bolt Diameter			Bolt Length		Wrench Size	Recommended Torque (Ft./Lbs.)
		Bolt Holes	Bolt Circle	Flange	Raised Face	Ring Joint
1/2	4	0.88	3-1/4	4-3/4	4	4-1/4	1-1/4	80
3/4	4	0.88	3-1/2	5-1/8	4-1/4	4-1/2	1-1/4	90
1	4	1	4	5-7/8	4-3/4	5	1-7/16	110
1-1/4	4	1	4-3/8	6-1/4	5	5	1-7/16	170
1-1/2	4	1.12	4-7/8	7	5-1/4	5-1/2	1-5/8	240
2	8	1	6-1/2	8-1/2	5-1/2	5-3/4	1-7/16	170
2-1/2	8	1.12	7-1/2	9-5/8	6-1/4	6-1/4	1-5/8	240
3	8	1.25	8	10-1/2	6-3/4	7	1-13/16	370
4	8	1.38	9-1/2	12-1/4	7-1/2	7-3/4	2	650
5	8	1.62	11-1/2	14-3/4	9-3/4	9-3/4	2-3/8	1000
6	12	1.5	12-1/2	15-1/2	10	10-1/2	2-3/16	820
8	12	1.75	15-1/2	19	11-1/4	11-3/4	2-9/16	1400
10	12	2	19	23	13-1/4	13-1/2	2-15/16	2100
12	16	2.12	22-1/2	26-1/2	14-3/4	15-1/4	3-1/8	2300
14	16	2.38	25	29-1/2	16	16-3/4	3-1/2	3400
16	16	2.62	27-3/4	32-1/2	17-1/2	18-1/2	3-7/8	4300
18	16	2.88	30-1/2	36	19-1/4	20-1/4	4-1/4	6200
20	16	3.12	32-3/4	38-3/4	21	22-1/4	4-5/8	7800
24	16	3.62	39	46	24	25-1/2	5-3/8	13000

 

Class 2500 Flanges

NPS	No. of Bolts	Bolt Diameter			Bolt Length		Wrench Size	Recommended Torque (Ft./Lbs.)
		Bolt Holes	Bolt Circle	Flange	Raised Face	Ring Joint
1/2	4	0.88	3-1/2	5-1/4	4-3/4	4-3/4	1-1/4	150
3/4	4	0.88	3-3/4	5-1/2	4-3/4	5	1-1/4	150
1	4	1	4-1/4	6-1/4	5-1/4	5-1/2	1-7/16	170
1-1/4	4	1.12	5-1/8	7-1/4	6	6	1-5/8	300
1-1/2	4	1.25	5-3/4	8	6-1/2	6-3/4	1-13/16	400
2	8	1.12	6-3/4	9-1/4	6-3/4	7	1-5/8	300
2-1/2	8	1.25	7-3/4	10-1/2	7-3/4	8	1-13/16	400
3	8	1.38	9	12	8-1/2	9	2	600
4	8	1.62	10-3/4	14	9-3/4	10-1/4	2-3/8	650
5	8	1.88	12-3/4	16-1/2	11-3/4	12-1/4	2-3/4	1500
6	8	2.12	14-1/2	19	13-1/2	14	3-1/8	2300
8	12	2.12	17-1/4	21-3/4	15	15-1/2	3-1/8	2400
10	12	2.62	21-1/4	26-1/2	19	20	3-7/8	4900
12	12	2.88	24-3/8	30	21	22	4-1/4	7900

FAQs

How Do I Find My Pipe’s Nps? 

See this helpful video: Piping basic – NPS, DN, Pipe Schedule

The NPS is usually engraved into the pipe itself. If the pipe is unmarked, you’ll have to manually measure the pipe with a string, a ruler, and a calculator. 

If your pipe’s thread is male, wrap a string around the pipe and mark the point of contact. 

Then, measure the string from the beginning to the end with a ruler and divide the measurement by 3.1459. 

The resulting measurement determines the pipe’s nearest DN or NPS value.  

If your pipe’s thread is female, simply measure the inside diameter of the pipe across the opening. 

Again, the resulting measurement determines the pipe’s nearest NPS value. 

What’s the Difference between a Male and a Female Thread Type? 

Male threads are situated outside the pipe, like a bolt. Female threads are situated on the inside, like a nut. Male threads are inserted into the female threads. 

To Sum Up

Flange bolts have a ton of important functions, especially in engine and transmission protection, plumbing, and large-scale applications. 

To prevent possibly catastrophic accidents, always consult a flange bolt size chart when installing a flange bolt.

The guide above provides you with all the measurements required for the job, including the recommended wrench and toque type.

 

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