PREN Calculator
Enter the chromium, molybdenum and nitrogen content of the grade in weight percent. The pitting resistance equivalent number is a comparison index between stainless grades, computed with the 16 N form used for duplex and austenitic steels.
PREN — pitting resistance equivalent number
PREN ranks pitting resistance between grades of the same family. It is a comparison index, not a guarantee of performance in a given service.
What this calculates
The pitting resistance equivalent number is a single figure that ranks stainless steels by how well their composition resists pitting in chloride environments. It weights the three elements that matter most: chromium counts once, molybdenum counts 3.3 times and nitrogen 16 times. A higher PREN means a grade that tolerates more chloride before pits start.
Engineers use it to sort candidates for a service: seawater, brine, chlorinated cooling water, coastal atmosphere. Common reference points are around 18 for 304, 24 for 316, 35 for 2205 duplex and above 40 for the super duplex and super austenitic grades. This page gives the figure for any composition you type in, including a mill certificate's actual analysis.
The number is an index, not a performance guarantee. Two grades with the same PREN can behave differently because of microstructure, surface finish, heat treatment and the welding they received. It ranks; it does not certify.
Show the formula and a worked example
The formula
- PREN = Cr + 3.3 × Mo + 16 × N (wt.%)
Dimensions in millimetres, density ρ in kg/m³ — as the engine runs them.
PREN = Cr + 3.3 × Mo + 16 × N, all in weight percent. This is the form in general use for austenitic and duplex stainless steels. A variant with a coefficient of 30 for nitrogen exists in some duplex literature; the engine uses 16 so results match the figures most specifications quote.
Blank fields count as zero. A ferritic grade with no molybdenum or nitrogen is handled correctly by entering only chromium. Negative values are refused, since a composition cannot be negative.
The worked example on this page runs a 22 Cr, 3.2 Mo, 0.18 N duplex composition, which is the workbook validation case for this calculation, and the engine returns 35.44.
Worked example
| Calculation | PREN — pitting resistance equivalent number |
|---|---|
| Chromium (Cr) | 22 wt.% |
| Molybdenum (Mo) | 3.2 wt.% |
| Nitrogen (N) | 0.18 wt.% |
| Theoretical result | 35.44 |
Computed by the calculation engine from these inputs at render time, rounded only for display.
Frequently asked
- 01Should I use the nominal composition or the certificate analysis?
- Use the certificate for a coil or plate you have, and the specification minimums for a grade you are comparing on paper. The minimums give the lowest PREN the grade is allowed to have, which is the safe basis for selection.
- 02What PREN do I need for seawater?
- Common practice treats 40 as the threshold for sustained seawater service, which puts the super duplex and 6 Mo austenitic grades in scope and leaves 316 out. Temperature, crevices, flow and chlorination all shift that line, so a specification or a corrosion engineer decides for a specific installation.
- 03Does tungsten count?
- Some formulations add 1.65 W for the tungsten-bearing super duplex grades. The engine's formula does not include tungsten; if your grade carries it, the true index is a little higher than shown.
- 04Is 316 with a higher PREN the same as 316L?
- No. The L designation is about carbon, which does not enter the PREN at all. Two heats of 316 and 316L with the same Cr, Mo and N have the same PREN and different weldability. Compare equivalents with care, and never treat them as identical.