What this calculator does
PCR primer melting temperature (Tm) works out estimated primer melting temperature from base composition (Wallace/Marmur-Doty). Enter your own figures above and the answer updates as you type: nothing is fixed in the code, so the result reflects exactly the numbers you supply.
The formula this calculator evaluates is printed under the tool and explained below, so you can check the working by hand or reuse it in a spreadsheet.
The formula
FormulaN<14 nt: Tm = 4(G+C) + 2(A+T) [Wallace rule]. N≥14 nt: Tm = 64.9 + 41·(G+C−16.4)/N [Marmur & Doty, 1962]
The inputs explained
| Field | What to enter |
|---|---|
| Count of A bases | A number. Starts at 6. |
| Count of T bases | A number. Starts at 6. |
| Count of G bases | A number. Starts at 5. |
| Count of C bases | A number. Starts at 5. |
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How the answer changes with count of a bases
Every other input is held at the calculator’s starting values while count of a bases varies. Select any row to load that scenario into the calculator.