This DNA concentration calculator converts A260 absorbance readings into concentration for dsDNA, ssDNA or RNA — and copy number per microliter for a molecule of known length.
Extinction constants assume the classic 1 cm pathlength and average base composition; purity rides along in the A260/A280 ratio (~1.8 for clean DNA, ~2.0 RNA) and A260/A230 (~2.0–2.2). Fluorometric assays (Qubit-style) measure only intact double-stranded DNA — expect them to read lower than UV on degraded or contaminated samples; both are “right” about different things.
Beer–Lambert, Calibrated by Convention
The constants (50/40/33 µg/mL per A260 unit) are the field’s agreed extinction averages — sequence extremes bend them by ~10–20%, GC content especially. Copy number additionally needs molecular weight (660 g/mol per bp) — molar thinking lives on our Molarity Calculator. Part of our Biology Calculators collection.
Worked Examples (Verified)
- A260 = 1.0, dsDNA (defaults): 50 µg/mL = 50 ng/µL; a 1,000 bp fragment at that concentration carries 4.56 × 10¹⁰ copies/µL
- Copy-number anchor: 1 µg of 1,000 bp DNA = 9.12 × 10¹¹ copies — the qPCR standard-curve constant hiding in application notes everywhere
- Same reading, different molecule: A260 1.0 reads 33 µg/mL as ssDNA and 40 as RNA — using the dsDNA constant on an RNA prep overstates by 25%
- Dilution reality: reading 0.8 on a 1:50 dilution means the stock is 0.8 × 50 × 50 = 2,000 µg/mL — the factor field exists because this multiplication is the most-skipped step at the bench
Purity & Practice
- A260/A280 < 1.7 suggests protein/phenol contamination; A260/A230 lows flag chaotropic salts/organics — quantification survives, downstream enzymes may not.
- Blank against the exact elution buffer; TE and water blank differently enough to matter at low concentrations.
- Below ~2 ng/µL, UV readings wobble — switch to fluorescence; above ~500 ng/µL, dilute into the spectrophotometer’s linear range.
- RNA preps: any genomic DNA contamination inflates A260 (DNA absorbs identically) — DNase and fluorometric RNA assays earn their keep.
Related Tools
- GC Content Calculator — the sequence behind the reading
- PCR Calculator — what the copies become
- Biology Calculators — the full collection
Frequently Asked Questions
How do I convert A260 to DNA concentration?
Multiply by the standard constant for your molecule: dsDNA ×50 µg/mL, ssDNA ×33, RNA ×40 — then by your dilution factor. A260 of 1.0 for dsDNA = 50 µg/mL (numerically equal to 50 ng/µL). Clean samples should show A260/A280 ≈ 1.8 (DNA) or 2.0 (RNA).
How many copies are in 1 µg of DNA?
Copies = (mass in ng × 6.022×10²³) ÷ (length in bp × 660 × 10⁹). For 1 µg of a 1,000 bp fragment: 9.12 × 10¹¹ copies. Halve the length, double the copies — copy number is why tiny plasmids dominate molar mixes.
Why does my Qubit read lower than my NanoDrop?
They measure different things: UV (NanoDrop) counts everything absorbing at 260 nm — free nucleotides, degraded DNA, RNA, contaminants; fluorescence counts intact double-stranded DNA specifically. A gap between them is diagnostic information about sample quality, not an instrument error.
What A260/A280 ratio is good?
~1.8 for pure DNA, ~2.0 for pure RNA. Lower suggests protein or phenol; check A260/A230 too (target ~2.0–2.2) for salt/organic carryover. Ratios bend at very low concentrations — a 5 ng/µL sample’s ratio is more weather than climate.
Does GC content change the A260 constant?
Slightly: extinction coefficients depend on base composition (purines absorb more), worth up to ~±15% on extreme-GC sequences. The 50 µg/mL constant assumes average composition; oligo work should use sequence-specific coefficients (the Primer Tm page’s world).
Is this DNA concentration calculator free?
Yes — free, instant, no sign-up.