Short answer
A working copy of OligoCalc runs at oligocalc.eu. It is Warren Kibbe's original code with the Northwestern copyright notice intact. If you want the exact tool you had bookmarked, use that. You can stop reading here.
If you specifically need magnesium and dNTP corrections, which OligoCalc does not do, there is a note on that further down.
What actually happened to the Northwestern URL
Three addresses have hosted OligoCalc over the years. This is the state of each one, checked on 31 July 2026.
| Address | State |
|---|---|
basic.northwestern.edu/biotools/oligocalc.html | Returns 404. The server still answers, but the whole biotools directory is gone, and the root returns 404 as well. There is no working HTTPS on that host. |
biotools.nubic.northwestern.edu | No DNS record at all. This host no longer exists, so links to it fail before a request is even made. |
oligocalc.eu | Working. Serves the OligoCalc interface and its documentation. |
The tool was not withdrawn or deprecated by its authors. What changed was the hosting. That is why old links in papers, protocols, and university resource pages fail while the tool itself is fine.
Which one you should use
These do different things, and for a lot of tasks OligoCalc is still the better answer. An honest comparison:
| OligoCalc (oligocalc.eu) | This site's Tm calculator | |
|---|---|---|
| Nearest-neighbor set | Sugimoto 1996 | SantaLucia 1998 unified |
| Sodium correction | Yes | Yes (Owczarzy 2004) |
| Magnesium correction | No, stated in its own docs | Yes (Owczarzy 2008) |
| dNTP chelation of Mg | No | Yes, solves for free Mg |
| Shows ΔH, ΔS, ΔG | No | Yes |
| Oligo molecular weight | Yes, DNA and RNA | No |
| OD260 and extinction coefficient | Yes | No |
| Hairpin / self-dimer | Yes, marked deprecated in its docs | Yes, base-pair screen only |
Use OligoCalc if
You want oligo molecular weight, OD260 to concentration conversions, or extinction coefficients. This site does not do those, and OligoCalc does them well. oligocalc.eu
Use this site's calculator if
You are calculating a Tm for a real PCR and want the magnesium and dNTPs in your buffer to affect the answer, or you want to see the ΔH, ΔS, and ΔG behind the number rather than just the Tm. DNA melting temperature calculator
For a 20-mer at 250 nM in 50 mM Na+, adding 1.5 mM Mg2+ and 0.8 mM dNTPs raises the calculated Tm by about 4 °C. A sodium-only figure reads low for PCR conditions.
FAQ
Why is OligoCalc not working?
The Northwestern address returns a 404 and the newer nubic address no longer resolves. The hosting changed, not the tool.
Where can I still use it?
oligocalc.eu, running the original code.
Does OligoCalc handle magnesium?
No. Its documentation states it makes no adjustment for magnesium or manganese. It takes sodium and primer concentration only.
Why do two nearest-neighbor calculators disagree?
OligoCalc uses the Sugimoto 1996 parameters. Most current tools use the SantaLucia 1998 unified set, so a degree or two of difference on identical inputs is expected.
Is this page affiliated with OligoCalc or Northwestern?
No. It is independent, and not endorsed by or connected to Northwestern University or the OligoCalc authors.
How this was checked
Every address in the table above was requested directly on 31 July 2026, with a DNS lookup and an HTTP request using a normal browser user agent, over both HTTP and HTTPS. A 404 means the host answered and reported no such page. "No DNS record" means the hostname did not resolve, so no request was possible. Feature claims about OligoCalc come from the tool and its own documentation at oligocalc.eu, including its statement that it makes no divalent cation adjustment.
Underlying methods: Sugimoto, N. et al., Nucleic Acids Research 24(22):4501–4505, 1996. SantaLucia, J., PNAS 95(4):1460–1465, 1998. Owczarzy, R. et al., Biochemistry 43(12):3537–3554, 2004, and 47(19):5336–5353, 2008.
If any of this goes out of date, or a link here breaks, let us know and it will be corrected.