Class 300 vs Class 600 Flanges
A clean doubling of pressure rating, bought almost entirely with flange thickness rather than diameter.
- Sizes in both classes
- 20
- Class 300 at 100 °F
- 740 psig
- Class 600 at 100 °F
- 1480 psig
- Pressure gain
- ×2.00
Short answer. Class 600 is rated 1480 psig at 100 °F against Class 300's 740 — exactly double. Notably, the flange outside diameter and bolt circle barely change: the extra capacity comes from about 28% more thickness and heavier bolting, not from a bigger flange.
What the class number means
A flange class is not a pressure. It is a rating series: the pressure a flange in that class may hold depends on the material group and the metal temperature, and it is read out of ASME B16.5 Table 2. The class number happens to equal the approximate working pressure in psig for Group 1.1 carbon steel at about 500 °F, which is where the numbers came from historically — but treating Class 300 as “300 psi” is wrong at every other temperature.
At ambient, Group 1.1 carbon steel is rated 740 psig in Class 300 and 1480 psig in Class 600 — a factor of 2.00. That ratio holds approximately across the temperature range, because every class in a group is derived from the same allowable stress curve.
| Temperature | Cl 300 · Gp 1.1 | Cl 600 · Gp 1.1 | Gain | Cl 300 · Gp 2.1 | Cl 600 · Gp 2.1 |
|---|---|---|---|---|---|
| 100 °F | 740 psig | 1480 psig | +100% | 720 psig | 1440 psig |
| 200 °F | 680 psig | 1360 psig | +100% | 600 psig | 1200 psig |
| 300 °F | 655 psig | 1310 psig | +100% | 540 psig | 1075 psig |
| 400 °F | 635 psig | 1265 psig | +99% | 495 psig | 995 psig |
| 500 °F | 605 psig | 1205 psig | +99% | 465 psig | 930 psig |
| 600 °F | 570 psig | 1135 psig | +99% | 440 psig | 885 psig |
| 650 °F | 550 psig | 1100 psig | +100% | 430 psig | 865 psig |
| 700 °F | 530 psig | 1060 psig | +100% | 420 psig | 845 psig |
| 750 °F | 505 psig | 1015 psig | +101% | 415 psig | 825 psig |
| 800 °F | 410 psig | 825 psig | +101% | 405 psig | 810 psig |
| 850 °F | 320 psig | 640 psig | +100% | 395 psig | 790 psig |
| 900 °F | 230 psig | 460 psig | +100% | 390 psig | 780 psig |
| 950 °F | 135 psig | 275 psig | +104% | 380 psig | 765 psig |
| 1000 °F | 85 psig | 170 psig | +100% | 355 psig | 710 psig |
Group 1.1 is A105 carbon steel; Group 2.1 is Type 304 stainless. Ratings are maximum allowable working gauge pressure at the metal temperature.
The flange gets bigger, thicker and more heavily bolted
Moving from Class 300 to Class 600 grows the flange outside diameter by about 5% and the thickness by about 28% on average across the 20 shared sizes. The bolt count rises in 1 of those sizes and the bolt diameter in 12. None of that is free: the flange costs more, the gasket costs more, the bolting costs more, and the extra bolt load has to be delivered with a torque wrench that can reach it.
Dimensions are given in inches, with the millimetre equivalent beneath in grey.
| NPS | Cl 300 OD | Cl 600 OD | Cl 300 thk | Cl 600 thk | Cl 300 BC | Cl 600 BC | Cl 300 bolting | Cl 600 bolting |
|---|---|---|---|---|---|---|---|---|
| NPS 1/2 | 3.7595.25 mm | 3.7595.25 mm | 0.5012.70 mm | 0.5614.22 mm | 2.6266.55 mm | 2.6266.55 mm | 4 × 1/2" | 4 × 1/2" |
| NPS 3/4 | 4.62117.35 mm | 4.62117.35 mm | 0.5614.22 mm | 0.6215.75 mm | 3.2582.55 mm | 3.2582.55 mm | 4 × 5/8" | 4 × 5/8" |
| NPS 1 | 4.88123.95 mm | 4.88123.95 mm | 0.6215.75 mm | 0.6917.53 mm | 3.5088.90 mm | 3.5088.90 mm | 4 × 5/8" | 4 × 5/8" |
| NPS 1 1/4 | 5.25133.35 mm | 5.25133.35 mm | 0.6917.53 mm | 0.8120.57 mm | 3.8898.55 mm | 3.8898.55 mm | 4 × 5/8" | 4 × 5/8" |
| NPS 1 1/2 | 6.12155.45 mm | 6.12155.45 mm | 0.7519.05 mm | 0.8822.35 mm | 4.50114.30 mm | 4.50114.30 mm | 4 × 3/4" | 4 × 3/4" |
| NPS 2 | 6.50165.10 mm | 6.50165.10 mm | 0.8120.57 mm | 1.0025.40 mm | 5.00127.00 mm | 5.00127.00 mm | 8 × 5/8" | 8 × 5/8" |
| NPS 2 1/2 | 7.50190.50 mm | 7.50190.50 mm | 0.9423.88 mm | 1.1228.45 mm | 5.88149.35 mm | 5.88149.35 mm | 8 × 3/4" | 8 × 3/4" |
| NPS 3 | 8.25209.55 mm | 8.25209.55 mm | 1.0626.92 mm | 1.2531.75 mm | 6.62168.15 mm | 6.62168.15 mm | 8 × 3/4" | 8 × 3/4" |
| NPS 3 1/2 | 9.00228.60 mm | 9.00228.60 mm | 1.1228.45 mm | 1.3835.05 mm | 7.25184.15 mm | 7.25184.15 mm | 8 × 3/4" | 8 × 7/8" |
| NPS 4 | 10.00254.00 mm | 10.75273.05 mm | 1.1930.23 mm | 1.5038.10 mm | 7.88200.15 mm | 8.50215.90 mm | 8 × 3/4" | 8 × 7/8" |
| NPS 5 | 11.00279.40 mm | 13.00330.20 mm | 1.3133.27 mm | 1.7544.45 mm | 9.25234.95 mm | 10.50266.70 mm | 8 × 3/4" | 8 × 1" |
| NPS 6 | 12.50317.50 mm | 14.00355.60 mm | 1.3835.05 mm | 1.8847.75 mm | 10.62269.75 mm | 11.50292.10 mm | 12 × 3/4" | 12 × 1" |
| NPS 8 | 15.00381.00 mm | 16.50419.10 mm | 1.5639.62 mm | 2.1955.63 mm | 13.00330.20 mm | 13.75349.25 mm | 12 × 7/8" | 12 × 1 1/8" |
| NPS 10 | 17.50444.50 mm | 20.00508.00 mm | 1.8145.97 mm | 2.5063.50 mm | 15.25387.35 mm | 17.00431.80 mm | 16 × 1" | 16 × 1 1/4" |
| NPS 12 | 20.50520.70 mm | 22.00558.80 mm | 1.9449.28 mm | 2.6266.55 mm | 17.75450.85 mm | 19.25488.95 mm | 16 × 1 1/8" | 20 × 1 1/4" |
| NPS 14 | 23.00584.20 mm | 23.75603.25 mm | 2.0652.32 mm | 2.7569.85 mm | 20.25514.35 mm | 20.75527.05 mm | 20 × 1 1/8" | 20 × 1 3/8" |
| NPS 16 | 25.50647.70 mm | 27.00685.80 mm | 2.1955.63 mm | 3.0076.20 mm | 22.50571.50 mm | 23.75603.25 mm | 20 × 1 1/4" | 20 × 1 1/2" |
| NPS 18 | 28.00711.20 mm | 29.25742.95 mm | 2.3158.67 mm | 3.2582.55 mm | 24.75628.65 mm | 25.75654.05 mm | 24 × 1 1/4" | 20 × 1 5/8" |
| NPS 20 | 30.50774.70 mm | 32.00812.80 mm | 2.4461.98 mm | 3.5088.90 mm | 27.00685.80 mm | 28.50723.90 mm | 24 × 1 1/4" | 24 × 1 5/8" |
| NPS 24 | 36.00914.40 mm | 37.00939.80 mm | 2.6968.33 mm | 4.00101.60 mm | 32.00812.80 mm | 33.00838.20 mm | 24 × 1 1/2" | 24 × 1 7/8" |
Thickness excludes the raised face. Bolting is quantity × nominal diameter in inches.
Same footprint, thicker flange
Class 600 shares its outside diameter and bolt circle with Class 400 in most sizes, and stays close to Class 300. What changes is thickness and bolt size. That makes the step attractive when space is tight: the flange occupies nearly the same envelope but carries twice the pressure. It also means the raised face jumps from 1/16 in to 1/4 in at Class 400 and above, so the gasket and the bolt length change even where the bolt circle does not.
Practical selection
Class 300 is enough when
- General process service under 740 psig at temperature
- Refinery and chemical plant lines at moderate pressure
- Low and medium pressure steam
- The spec break sits above the operating pressure with margin
Class 600 is worth it when
- High-pressure steam and boiler feedwater
- Hydroprocessing, gas compression and pump discharge headers
- Where relief valve set pressure pushes the design above Class 300
- Where the 1/4 in raised face and heavier bolting are wanted for joint stiffness
Dimensions in full: Class 300 weld neck · Class 600 weld neck · Class 300 blind · Class 600 blind · all P-T ratings · how derating works.
Common questions
Why is Class 600 exactly twice Class 300?
Because the classes in a group are derived from the same allowable stress by a fixed set of multipliers. The 300/600 pair falls at a clean factor of two; other pairs do not.
Is Class 400 used much?
Rarely. Most piping specs jump 300 to 600 because Class 400 shares Class 600's outside diameter and bolt circle while offering only two-thirds the rating, so it saves little and complicates stocking.
Does the raised face change between Class 300 and 600?
Yes. Classes 150 and 300 use a 1/16 in raised face; Class 400 and above use 1/4 in. The gasket and the bolt length must both account for it.
Can Class 600 flanges use ring type joints?
Yes. RTJ is available across the classes and becomes the common choice at Class 600 and above for high-temperature and hydrocarbon service.