DNA Molecular Weight Calculator
Paste a sequence, get the mass: single strand, double strand via the complement road, GC content and the 5′-phosphate door — the OligoCalc grammar, shown working.
DNA Molecular Weight Calculator
Results recalculate instantly on every keystroke. Nothing you type is transmitted.
What this result does not account for
- Unmodified A-C-G-T sequences only
- Table masses conventional, quoted approximate
In short: Eight bases, two of each: ATCGATCG prices at 2,409.640000 Da single-stranded — the four nucleotide masses sum to 2,471.600000 and the 5′-hydroxyl adjustment takes 61.960000 back — and 4,819.280000 double-stranded by walking the complement (A↔T, G↔C) and subtracting both termini. GC content is 50.000000%: two G or C in eight. Restriction enzymes leave a 5′-phosphate behind: 79.000000 more on the single strand.
Formula
ss = A·313.21 + T·304.2 + C·289.18 + G·329.21 − 61.96 · ds = strand + complement − 2×61.96 · +79.0 with a 5′-phosphate
The page weighs the OligoCalc way: each nucleotide carries its table mass as the monophosphate, the strand sums them, and a single terminal adjustment of 61.96 converts the 5′ end from phosphate to hydroxyl — the state of synthesized oligonucleotides. The double-stranded figure is not twice the single: it is the strand plus its complement, each carrying its own terminal adjustment. The nucleotide masses are conventional table values, quoted approximate.
Worked Example
- Paste or type the sequence — spacing is formatting, not content.
- Read the single-strand mass and the terminal adjustment that shaped it.
- Read the double-strand mass from the complement road.
- Check GC content; add the phosphate card if your fragment was cut, not synthesized.
Defaults: ATCGATCG → ss 2,409.640000 Da, ds 4,819.280000 Da, GC 50.000000%. GATTACA → ss 2,104.460000 Da, GC 28.571429%. The four-base ATCG → 1,173.840000 ss, 2,347.680000 ds.
Strengths & Limits Of This Model
Where this engine is strong
- ss and ds priced by different honest roads
- The 5′-phosphate door printed as a card
Where it stops
- No ambiguity codes or modifications
- No Tm — mass only
Practical Use Cases
Oligo ordering
mass on the purchase order
Cloning
fragment weight for molar conversions
Teaching
the terminal adjustment made visible
Methodology & Editorial Standards
Computation runs in IEEE-754 double precision at full internal precision; rounding to two decimal places occurs strictly at the display layer, so no cumulative drift enters the result. All monetary outputs use accounting presentation — grouped thousands, two decimals, negatives in parentheses — so figures can be transcribed directly into a model or working paper. Division-by-zero and out-of-domain inputs return an em-dash rather than a misleading number.
This engine was reconciled against an independent reference implementation and hand-verified for the worked example above before release. Our full five-stage review process is published on the About Us page.
Disclaimer. This calculator is provided for informational and modelling purposes only and does not constitute financial, tax, legal, medical, or engineering advice. Verify all figures with a qualified professional before acting on them.
DNA Molecular Weight Calculator — 8 Expert FAQs
8 analyst-written answers to the questions practitioners actually ask — optimised for voice and answer-engine retrieval.
Why does the mass subtract 61.96 instead of adding water?
Because the table masses are nucleotide MONOPHOSPHATEs — what an oligo is assembled from — and a synthesized strand ends in hydroxyls, not a phosphate. One adjustment of 61.96 converts the 5′ terminus. It is a single terminal term, not a per-bond water subtraction; longer sequences do not subtract more.
When do I add the 79?
When the fragment was cut by restriction enzymes or otherwise carries a 5′-phosphate — the card prints the adjusted figure so the door is a glance, not a recalculation. Primer-extension strands keep their hydroxyl and stay at the plain figure. A phosphorylated duplex pays it on both strands.
How is the double-strand mass computed?
Not by doubling. The complement strand swaps A for T and G for C, so its mass differs unless the sequence is a palindrome in counts. The page prices strand plus complement and subtracts both terminal adjustments. For ATCG the two strands happen to match in composition and the ds figure is exactly twice minus 123.92.
Why does GC content matter enough to print?
It is the cheapest cross-check in molecular biology: a sequence’s GC percentage shapes melting behaviour and confirms the paste. The page counts G and C over the length — 4 of 8 in the default, 2 of 7 in GATTACA — so a misclicked row shows up before the mass is trusted.
What about modified bases or mixed alignments?
Outside the grammar, honestly: the page refuses letters outside A-C-G-T by name and position rather than guessing. IUPAC ambiguity codes (R, Y, N), phosphorothioate marks and degenerate mixes each change the mass in ways a four-letter table cannot price silently.
What does the ds card add on top of doubling?
The truth that doubling is usually wrong: the complement strand swaps A for T and G for C, so its mass differs unless the composition is symmetric. The card prints strand mass, complement mass and both terminal adjustments separately — for ATCGATCG the two strands coincide in composition and ds lands at exactly twice minus 123.920000, but GATTACA would expose any lazy doubling instantly.
Why does the page echo the sequence back?
Proof of reading. The card shows the surviving letters in parentheses for short pastes, so a truncated clipboard, a stray line break or an autocorrected character surfaces before the mass gets copied anywhere. Long sequences skip the echo and rely on the letter counts in the GC card to show what was actually weighed.
How is this different from the molecular weight page?
Input grammar. The molecular weight page parses a chemical formula — element symbols with counts. This page counts letters of a biological alphabet and prices them from a nucleotide table with terminal conventions. A sequence is not a formula; writing ATCG into the formula parser would try to read arsenic, tellurium and carbon.