Why is 100 µM the default?

Because that is what the source recommends as standard: it is simple to make and versatile for the dilutions that follow. It is only a recommendation, though — type a different value and the table recomputes against it.

Can I resuspend in water?

The source prefers TE buffer because it holds the pH steady, and offers nuclease-free sterile water as the alternative. It warns against DEPC-treated water and water taken straight from a deionizer: both are often acidic, which damages DNA oligos over time.

How many can I do at once?

There is no limit. Paste a whole 96-well plate if you like. That is the reason this page exists as its own tool — the arithmetic is a single division, but what arrives is never one oligo, it is a plate of them.

It says a line could not be read as a number.

Almost always that is the header row copied along with the data. It is left out of the calculation and reported by line number, so you can leave it where it is.

Can I paste numbers without names?

Yes — a line with a single value is read as the amount. Names are worth including, though: “91 µL into row 3” is not a usable instruction in front of a rack of twelve identical tubes.

Oligo Resuspension Calculator

Paste a whole delivery list and get the buffer volume for every tube at once, with clipboard and Excel export.Paste the list exactly as it arrived. This tool exists to do a plateful at a time.

Oligo list

One per line, as “name ⇥ amount in nmol”. Copying two columns out of a spreadsheet pastes in exactly. A line with only a number works too.

Unit of the amount delivered

Target stock concentration (µM)

Making a working solution

Working concentration (µM)

Working volume to make (µL)

That is a 10× dilution: 10 µL of stock plus 90 µL of diluent makes 100 µL.

Source: IDT, “Expert tips for resuspending oligos and primers” — recommends 100 µM as the standard stock and says to add ten times the nmol figure in µL.

How to use

Paste the list of oligos you received. Copying the name and amount columns together out of a spreadsheet pastes in as-is.
Choose the unit of the delivered amount. For nmol the tube label is enough; for µg each line needs a molecular weight in a third cell.
Check the target stock concentration. The 100 µM default is what the source recommends as standard.
The table gives the buffer volume for every tube. Copy it or download it as Excel and take it to the bench.
The section below works out the working solution as well — that, not the stock, is what goes into a reaction.

Oligo Resuspension Formula and practical notes

µM means micromoles per litre, and scaled down that is exactly picomoles per microlitre. So there is no constant to remember here: a 100 µM stock simply has 100 pmol in every µL.

The formula

One line if the amount is in nmol; µg needs the molecular weight to reach nmol first.

Buffer to add (µL): amount in nmol × 1000 ÷ target in µM
µg to nmol: amount in µg × 1000 ÷ molecular weight
Working solution: stock needed (µL) = working × volume ÷ stock concentration

Practical notes

This matches the source's own worked example: 9 nmol taken to 100 µM needs 90 µL — ten times the nmol figure, in µL.
Spin the tube down briefly before opening it. Dried oligo is a thin film or a white flaky pellet, and a pellet dislodged in transit can fly out when the cap comes off.
If it will not dissolve, the source suggests warming to 55°C for 1–5 minutes and vortexing thoroughly. Oligos carrying fluorophores or hydrophobic modifications take longer.
⚠️ This same calculation also lives inside the primer Tm tool. The two compute it separately, so changing the formula in one does not change the other. If the numbers ever disagree, suspect that first.
OD260 is not accepted as an input. Turning A260 into moles needs that oligo's ε260, and the specification sheet carrying that value already prints the amount in nmol — so OD is the worse route. No primary source for a table of ε260 values was obtained either.