DNA & RNA Calculators

Before you can amplify, clone or quantify a nucleic acid, you usually need to know something about its composition. How GC-rich is this sequence? Will it fold on itself? How much does a given length weigh, and how concentrated is the stock on the bench? These are sequence-analysis questions, and they are answered by counting and simple arithmetic — but the rules for counting matter, especially when a sequence carries ambiguous bases. This collection covers nucleic-acid composition from first principles: every tool states its method, works through real sequences, and cites its sources, and none is tied to a particular vendor’s platform.

GC Content & Base Composition

GC content is the most-quoted number about a sequence, because G·C pairs stack more stably than A·T pairs and therefore drive melting temperature, secondary structure and amplification stringency. The GC content calculator returns GC%, AT%, the exact per-base counts, the length and the GC:AT ratio, and it handles RNA (U read as a weak, A/T-like base) and IUPAC degenerate codes honestly — S counts as GC, W counts as AT, and every other ambiguous code is reported separately rather than guessed, so a sequence full of Ns is never silently called AT-rich. It also gives a low / typical / high call calibrated to the 40–60% window primer designers aim for. To understand the counting rules and why they matter, the how to calculate GC content guide walks through the arithmetic with worked examples, including sequences with degenerate bases.

From Composition to Melting Behaviour

Composition is a summary; behaviour is what you actually care about at the bench. Because GC content is the biggest single driver of melting temperature, the natural next step after counting bases is to predict Tm. The Primer Tm calculator takes the same sequence and returns the SantaLucia 1998 nearest-neighbour melting temperature, the recommended annealing temperature and a gradient window — which is why the two tools are built to be used together and cross-linked. That calculator also anchors the PCR & qPCR set, where the melting number feeds directly into a cycling program.

Molecular Weight & Concentration

A sequence of known length and composition has a predictable molecular weight, and a weighed or measured amount of it has a predictable molar and mass concentration — the numbers you need to normalise a template or a primer stock. The DNA molecular weight calculator returns the exact free-acid mass from a sequence (summing per-base residue masses with the terminal-group correction), or a ~309 g/mol-per-nt / ~618-per-bp average estimate when you only know the length, and reports the µg/nmol and nmol/µg conversions alongside the base counts. Feed that molecular weight into the DNA concentration calculator, which turns an A260 reading into µg/mL for dsDNA, ssDNA or RNA using the standard extinction factors, honours the dilution factor and path length, and — when you supply a molecular weight — reports the molar concentration in nM too. For the general stock-to-working conversions behind these, the laboratory calculators still apply: the molarity calculator turns a mass and volume into a concentration once you have a molecular weight.

Which calculator do I need?

Start from what you are holding, not from the tool name:

• A DNA or RNA sequence and you want its composition → GC content calculator for GC%, AT%, base counts, the GC:AT ratio and the low/typical/high call.
• The same sequence and you need its melting behaviour → Primer Tm calculator for Tm, annealing temperature and gradient window.
• A sequence with N or other degenerate bases → the GC content calculator, which reports ambiguous bases separately so GC% stays a defensible floor; the how to calculate GC content guide explains the rules.
• A sequence or length and you need its mass → DNA molecular weight calculator for the exact free-acid MW, the µg/nmol and nmol/µg conversions and the base counts.
• An A260 reading and you need the concentration → DNA concentration calculator for µg/mL (dsDNA, ssDNA or RNA) with dilution and path-length correction, plus molarity when you supply a MW.

When two tools seem to fit, prefer the one whose inputs match what you actually measured: a sequence string goes to a composition, Tm or molecular-weight tool, and a spectrophotometer absorbance goes to the concentration tool.

DNA & RNA Tools

• GC Content Calculator — GC%, AT%, base counts, GC:AT ratio and degenerate-base handling for DNA and RNA.
• Primer Tm Calculator — melting and annealing temperature from the same sequence, by nearest-neighbour or Wallace.
• DNA Molecular Weight Calculator — exact free-acid MW from a sequence, or a length-based average, with µg/nmol and nmol/µg.
• DNA Concentration Calculator — A260 to µg/mL for dsDNA, ssDNA or RNA, with dilution, path length and optional molarity.