Calculator
Example data table
| Scenario | Input DNA amount | Type & length | Estimated molar mass | Mass result |
|---|---|---|---|---|
| Plasmid-like dsDNA | 2 pmol | dsDNA, 3000 bp | ~1.98×106 g/mol | ~3.96 µg |
| Short ssDNA oligo | 50 nmol | ssDNA, 20 nt | ~6600 g/mol | ~330 µg |
| From concentration × volume | 10 µM × 25 µL | dsDNA, 500 bp | ~3.30×105 g/mol | ~82.5 ng |
Formula used
Core relationship: mass (g) = moles (mol) × molar mass (g/mol)
If you use concentration × volume: moles = concentration (M) × volume (L)
If you estimate molar mass from length: MW ≈ length × factor, where factor is ~660 g/mol per bp for dsDNA, ~330 g/mol per nt for ssDNA, and ~340 g/mol per nt for RNA.
How to use this calculator
- Choose an input mode: moles, concentration × volume, or copies.
- Enter your values and select the correct units.
- Pick a molar mass method: estimate from length or enter directly.
- For length-based estimates, select polymer type and length.
- Click Calculate to see results above the form.
- Use the CSV/PDF buttons in the results panel to export.
Lab notes and practical guidance
1) What this calculator actually converts
In molecular biology, “mass” is what you weigh or pipette, while “moles” represent molecule count. This tool converts a DNA amount (in mol, pmol, or from concentration × volume) into mass using a molar mass estimate. It is ideal for planning dilutions, preparing standards, or checking recovery after cleanup.
2) The core relationship and units
The core relationship is mass = moles × molar mass. The calculator keeps internal units in mol, liters, and grams. Concentration is converted to mol/L (M), volume to L, and output mass can be displayed as g, mg, µg, ng, or pg. Small samples often look clearer in ng or pg.
3) Length-based molar mass factors you should know
When you do not have a vendor-provided molecular weight, the calculator estimates molar mass from length using average base factors. Typical values are about 660 g/mol per base pair for double‑stranded DNA, about 330 g/mol per nucleotide for single‑stranded DNA, and about 340 g/mol per nucleotide for RNA. These averages are widely used for quick lab math.
4) Example: dsDNA plasmid-scale estimate
A 3000 bp dsDNA molecule has an estimated molar mass near 3000 × 660 = 1.98×106 g/mol. If you have 2 pmol, that is 2×10−12 mol, giving a mass near 3.96×10−6 g, or about 3.96 µg. This matches typical mini‑prep yields for small elutions.
5) Concentration × volume is the everyday workflow
Many tubes are labeled in molar units such as 10 µM or 50 nM. If you pipette 25 µL of a 10 µM dsDNA solution, moles are 10×10−6 × 25×10−6 = 2.5×10−10 mol. Combine that with your molar mass to get mass immediately, without separate unit conversions.
6) Copy count option for standards and qPCR
If your workflow reports “copies,” the calculator converts copies to moles using Avogadro’s constant 6.02214076×1023 molecules/mol. This is useful for preparing DNA standards, spike‑ins, and sanity‑checking copy-based concentration claims.
7) When to use a custom factor
Average base factors are estimates. Use a custom factor when you have a more accurate molecular weight, modified bases, unusual composition, or labeled oligos. Vendors may provide exact molecular weight in Da or g/mol; entering that directly can reduce bias, especially for short oligos.
8) Common pitfalls and quick checks
Confirm whether your length is in bp (dsDNA) or nt (ssDNA/RNA). Ensure concentration units match the label (µM vs nM). If results look off by 1000×, check mL vs µL. For very large constructs, consider reporting in µg or mg to avoid tiny exponents.
FAQs
1) Is the length-based molar mass exact?
It is an estimate using average base factors. Exact molar mass depends on base composition and modifications. For vendor oligos or labeled sequences, enter the provided molecular weight for best accuracy.
2) Should I enter bp or nt for my sequence?
Use bp for double‑stranded DNA length. Use nt for single‑stranded DNA or RNA. If you choose the wrong unit family, the calculator may produce a molar mass that is roughly twofold or more off.
3) Why does mass change when I switch dsDNA to ssDNA?
The average mass per base unit differs. dsDNA uses an average per base pair, while ssDNA uses per nucleotide. For equal “length numbers,” the implied molecular weight and mass can differ substantially.
4) How do I convert a tube labeled 20 µM into mass?
Select concentration × volume mode, enter 20 µM, then enter the volume you will use. Choose length-based or direct molar mass, then read the mass in your preferred unit.
5) What if I only know molecular weight in kDa?
Use the direct molar mass method and choose kDa. The calculator converts kDa to g/mol internally (1 kDa ≈ 1000 g/mol), then computes mass from your moles.
6) Can I use this for RNA standards?
Yes. Choose RNA under polymer type and enter length in nt or knt. If you have an exact molecular weight from a supplier, enter it directly for higher accuracy.
7) Why does my output show scientific notation?
Very small or very large numbers are displayed in scientific notation to stay readable. Switch output units (for example, from g to ng) to get a more familiar magnitude for tiny samples.