# Oligonucleotide Tm Calculator (Salt-Corrected) Classic Tm rules for DNA oligos: Wallace 2(A+T)+4(G+C) under 14 nt and the Sambrook/Primer3 salt formula 81.5 + 16.6·log₁₀[Na⁺] + 0.41×(%G+C) − 600/n from 14 nt, with auto switching, GC%, composition, and both estimates side by side. Wallace 1979 / Sambrook 1989 derived. Educational use only. > Canonical page: https://elysiatools.com/en/tools/oligo-melting-temperature - **Category:** Education - **Keywords:** oligo tm calculator, melting temperature salt corrected, wallace rule tm, tm 81.5 16.6 log sodium, oligonucleotide gc content, primer tm estimation, hybridization temperature, short oligo tm, sambrook tm formula, molecular biology, lab calculator ## Overview The Oligonucleotide Tm Calculator estimates DNA oligo melting temperature using the Wallace rule for short sequences or a salt-corrected formula for longer sequences. Enter a 5′→3′ DNA sequence, choose the calculation method, and provide sodium concentration to view Tm, GC percentage, length, base composition, and both estimates. ## Inputs - **Oligonucleotide Sequence (5'→3')** (text): DNA sequence, A/C/G/T only (spaces stripped, case-insensitive), 4–200 nt. - **Tm Method** (select): Auto uses Wallace below 14 nt and the salt-corrected formula from 14 nt up. - **Sodium Concentration (mM)** (number): Total \[Na⁺\] (or equivalent monovalent) of the buffer; used by the salt-corrected formula. - **Decimal Places** (number) ## When to use - Estimate the melting temperature of a short DNA oligonucleotide or probe. - Compare Wallace and salt-corrected Tm estimates for the same sequence. - Check how GC content, oligo length, and sodium concentration affect an approximate Tm. ## How it works - Enter a DNA sequence containing only A, C, G, and T; spaces are removed and letter case is ignored. - Choose Auto, Wallace rule, or the salt-corrected formula, then enter sodium concentration in mM. - Auto mode uses the Wallace rule below 14 nt and the salt-corrected formula from 14 nt upward. - The JSON result reports the selected Tm estimate, method used, GC percentage, sequence length, composition, and both calculated estimates. ## Use cases - Quickly estimate hybridization temperatures for short DNA probes. - Review GC content and base composition while comparing oligo candidates. - Explore the effect of sodium concentration on a longer oligonucleotide’s approximate Tm. ## Frequently asked questions ### What sequence can I enter? Enter a DNA oligonucleotide in the 5′→3′ direction using A, C, G, and T only. The allowed length is 4–200 nt. ### Which Tm method does Auto use? Auto uses the Wallace rule for sequences shorter than 14 nt and the salt-corrected formula for sequences of 14 nt or longer. ### Why is sodium concentration required? Sodium concentration is used by the salt-corrected formula and should represent total sodium or equivalent monovalent ion concentration in the buffer. ### What formula is used for the salt-corrected estimate? The calculator uses 81.5 + 16.6×log₁₀[Na⁺] + 0.41×(%G+C) − 600/n, with sodium concentration converted from mM to M. ### Are these Tm values suitable for final primer design? 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