H40, J55, N80, L80, P110, Q125 — API 5CT casing and tubing grades read like a code because, functionally, they are one: a compact way of specifying a pipe’s strength, and in some cases its metallurgy, without writing out a full material spec every time. Here’s what the letters and numbers actually mean, and where they stop telling you enough.
What the number means
The number in an API 5CT grade is the minimum yield strength in ksi (thousands of psi) — N80 has a minimum yield of 80,000 psi, Q125 has a minimum yield of 125,000 psi. This number feeds directly into burst, collapse, and tension rating calculations; it’s the SMYS (specified minimum yield strength) value used in the API TR 5C3 burst and collapse equations.
What the letter doesn’t mean
This is the part that trips people up: the letter prefix has no consistent logic across grades. It doesn’t encode chemistry, sour service suitability, or manufacturing method in a predictable pattern — it’s simply a historical designation assigned as each grade was introduced. You cannot infer a grade’s properties from its letter; you have to know the specific grade.
Common grades at a glance
- H40 — 40 ksi minimum yield, lowest common grade, limited to low-pressure, shallow, non-critical applications.
- J55 / K55 — 55 ksi minimum yield, widely used for surface and intermediate casing in moderate-pressure wells.
- N80 — 80 ksi minimum yield, a workhorse grade for production casing and tubing in standard service.
- L80 — 80 ksi minimum yield like N80, but with a tighter hardness specification qualifying it for sour (H2S) service per NACE MR0175.
- P110 — 110 ksi minimum yield, used where higher pressure or deeper wells require more strength than N80/L80 can provide.
- Q125 — 125 ksi minimum yield, among the highest-strength common grades, used in high-pressure, high-depth, or high-load applications; sour service qualification depends on the specific product and heat treatment.
Why L80 exists next to N80
L80 is a useful example of the letter/number split: it has the identical minimum yield strength to N80, but a different, tighter hardness control specifically because sulfide stress cracking risk increases with material hardness. Same nominal strength, different metallurgical qualification — which is exactly why grade selection can’t be reduced to “pick the yield strength you need.” See our post on NACE MR0175 and sour service for how that qualification actually works.
Using grade in a real design
Once a grade (and therefore SMYS) is selected against the well’s pressure, depth, and sour-service requirements, it becomes a direct input to burst and collapse calculations. Try our calculators with a specific grade’s SMYS value: Internal Yield (Burst) Pressure and Collapse Pressure.
Need help selecting casing or tubing grade for a specific well design? Talk to an engineer.
