Oligo Resuspension Calculator (nmol to µM)
Find how many µL of water or TE buffer to add to a dried oligo or primer (nmol on the spec sheet) for a 100 µM stock, its ng/µL, total µg, and how to make a 10 µM working dilution.
How it works
volume (µL) = nmol ÷ desired µM × 1,000 · mass (µg) = nmol × MW ÷ 1,000 · ng/µL = µM × MW ÷ 1,000 · working dilution: stock (µL) = working µM × working volume ÷ stock µM
Synthetic oligonucleotides arrive dried with the yield in nanomoles on the specification sheet. Because 1 µM is 1 pmol/µL, the volume that gives a 100 µM stock is simply nmol × 10 µL. Spin the tube before opening, dissolve in nuclease-free water or low-EDTA TE (pH 7.5–8, which keeps oligos stable longer), vortex and let it stand a few minutes. Most labs keep a 100 µM master stock frozen and use a 10 µM working dilution so that the master is thawed rarely.
Worked example
An oligo of 25.3 nmol resuspended to 100 µM needs 25.3 ÷ 100 × 1,000 = 253.0 µL of water or TE. With a molecular weight of 6,100 g/mol it weighs 154.33 µg and the stock is 610.0 ng/µL. For 100 µL of 10 µM working solution mix 10.00 µL of stock with 90.00 µL of water.
Sources: IDT — Expert tips for resuspending and diluting oligos and primers · IDT — How to store oligonucleotides for greatest stability
Frequently asked questions
How do I resuspend primers to 100 µM?
Multiply the nmol on the tube by 10 and add that many µL: 25 nmol → 250 µL, 38.6 nmol → 386 µL. Use nuclease-free water or TE buffer, vortex, spin down and store at −20 °C.
Should I use water or TE buffer?
TE (10 mM Tris, 0.1–1 mM EDTA, pH 8) protects oligos from acid hydrolysis and nucleases during long storage. Water is fine for short-term working stocks or when EDTA would interfere with downstream enzymes.
How do I convert µM to ng/µL for an oligo?
ng/µL = µM × molecular weight (g/mol) ÷ 1,000. A 20-mer of about 6,100 g/mol at 100 µM is about 610 ng/µL.
What working concentration should primers be?
Commonly 10 µM, so that 1 µL in a 20–50 µL reaction gives 0.2–0.5 µM. Probes for qPCR are often kept at 5–10 µM.
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This calculator is for educational purposes. Double-check important results before acting on them.