The standard 2/10 net 30 supplier discount calculation is 2 percent of the discountable goods subtotal, before sales tax and before freight, if the invoice is paid within 10 days. It is not 2 percent of the invoice total. Two conventions, two different numbers, and almost nobody in a small shop checks which one their supply house is using.
The gap is small per invoice and large per year. On a $9,400 material invoice with tax and freight on top, the difference between the two methods is roughly $60. Run $40,000 a month through that and you are arguing about real money by December.
Both errors happen. A supply house that computes the discount on a base narrower than your agreement says shorts you. A shop that pays a flat 98 percent of every invoice total shorts the supply house, then spends the next quarter fielding calls about penny balances. This post is the arithmetic for both, plus the specific things that quietly change the base.
What 2/10 net 30 actually means
Three parts:
- 2 is the discount percentage.
- 10 is the number of days from the invoice date you have to pay to earn it.
- net 30 is the date the full amount is due if you do not.
The convention in the trades is that the discount computes on the merchandise subtotal, before sales tax and before freight. That is what contractors describe when they compare notes on prompt-pay terms, including a long thread on the Mike Holt forums about a distributor’s 2 percent discount and how it gets calculated. Nothing in that convention is a regulation. It is contractual and customary, which means your account agreement controls, and if your agreement does not say, nobody can tell you that you are definitively right.
That is the single most useful thing to know here: get it in writing. Ask your rep for the terms language on your account, specifically what base the discount applies to and when the clock starts.
The arithmetic, worked
Say a delivered invoice looks like this. The numbers are illustrative.
| Line | Amount |
|---|---|
| Merchandise subtotal | $9,400.00 |
| Freight | $185.00 |
| Sales tax (7% on merchandise) | $658.00 |
| Invoice total | $10,243.00 |
Now the two calculations:
| Method | Base | Discount | You pay |
|---|---|---|---|
| Correct: 2% of merchandise subtotal | $9,400.00 | $188.00 | $10,055.00 |
| Wrong: flat 98% of invoice total | $10,243.00 | $204.86 | $10,038.14 |
The flat 98 percent habit takes $16.86 more discount than the terms give you. That is an open balance on your account, not a windfall. Somebody at the distributor will chase it, and enough of them will get your prompt-pay privileges reviewed. The standard AP response to a mismatch is to withhold payment and request a corrected invoice or a credit note, not to self-adjust the payment and hope it nets out.
The mirror error is the one that costs you. If the supply house computes the 2 percent on a base that excludes items your agreement says are discountable, or applies it after netting an unrelated credit, your discount comes in under $188 and the shortfall never appears as a line anyone can see. It shows up as an invoice that was “paid short” or “paid over” and then gets adjusted quietly at month end.
What the discount costs you if you skip it
Worth doing this arithmetic once, because it reframes the whole question.
Taking 2/10 means giving up 2 percent to pay 20 days earlier than you had to. Expressed as an annualized rate: you are paying 2 divided by 98, or about 2.04 percent, to hold the money for 20 extra days. There are about 18.25 twenty-day periods in a year. That is roughly 37 percent on a simple annualized basis. This is arithmetic, not a market statistic, and your real number depends on your terms.
Almost no line of credit costs a contractor 37 percent. So if you can fund it, taking the discount is nearly always right, and the interesting question stops being “should we” and becomes “did we actually get it.”
That framing also explains why distributors offer it at all. Distributors carry genuine working capital strain: on the rebate side, they pay higher upfront cost before receiving manufacturer reimbursements, and one regional distributor freed up over $2,000,000 by clearing claims aged past 90 days. Cash in ten days is worth something real to them. Prompt-pay terms are a trade, not a favor.
Six things that quietly change the base
Sales tax. The discount should not compute on tax. Whether the tax itself should be recalculated on the discounted merchandise amount is a state-by-state question, and it is a genuine one. Ask your accountant rather than your counter guy, and do not assume the invoice got it right in either direction.
Freight and delivery charges. Normally outside the discountable base. If your invoices bundle freight into the merchandise subtotal rather than showing it separately, the base is wrong before anyone applies a percentage to it. This is worth a specific ask: separate freight line, every invoice.
Non-discountable categories. Some agreements carve out categories entirely: special order, direct ship, non-stock, cut wire, rental. If your terms have carve-outs, an invoice heavy in those items will produce a discount that looks short but is not. You need the carve-out list in writing to tell the difference.
Credits applied against the invoice. A return credit or a price-adjustment credit that lands before payment reduces the base. That is correct. What is not correct is a credit reducing the base while the original full-price line still shows, which is how a discount silently gets computed twice on the same dollars. If your credits have a habit of arriving late or not at all, that is its own problem, and the fix is documented in the credit request email template.
Statement terms versus invoice terms. Many supply houses run prompt pay off a monthly statement rather than per invoice: pay the whole prior month’s statement by a set date and take the discount on all of it. That changes the clock dramatically, and it means a single disputed invoice can jeopardize the discount on the entire statement. If you are on statement terms, know the cutoff date and know whether a disputed line kills the whole discount or just its own.
Duplicate invoices in the run. If the same invoice was issued twice and you pay both inside the discount window, you have now taken a discount on money you should never have paid at all, which makes the duplicate harder to spot in reconciliation, not easier. The detection method is in catching a duplicate supply house invoice.
How to check what you actually got, in ten minutes
You need one month of invoices and the corresponding remittances or statements.
- Pull the terms language. Written, from your rep. Discount percent, day count, what the base excludes, and whether the clock runs from invoice date, delivery date, or statement date.
- Pick five invoices you paid inside the window. Ideally a mix: one delivered, one will-call, one with freight, one with a credit applied, one special order.
- Recompute the discount by hand on each one: discountable merchandise subtotal times your rate. Write the number down before you look at what was taken.
- Compare to what was actually applied. On the statement, the remittance, or the credit memo. Note the direction of any difference.
- Check the day count on each. Count from the date the terms say the clock starts, not from the date you opened the envelope. Paper invoices that sat in a truck for a week are the number one reason a discount was legitimately lost, which is the practical case for getting paper invoices scanned the day they arrive.
- Look at the freight line. Present and separate, or buried in the subtotal?
- Confirm the invoice matched the PO and the packing slip first. A discount computed on a quantity you never received is still an overpayment. That check is three-way matching, and it belongs before the discount math, not after.
If four of five agree with your hand calculation, you have a clean process and you can spot-check quarterly. If they do not agree, and they disagree in the same direction every time, you have found a rule in someone’s ERP rather than a mistake by a person, and that is a conversation with your rep, not with AP.
How to raise it without making it a fight
Keep it arithmetic. This is the easiest category of billing question to resolve because there is no judgment involved: either the base was right or it was not.
Send the invoice number, the merchandise subtotal you calculated the discount on, the rate from your terms, the discount you expected, the discount that was applied, and the difference. Five facts, three sentences. Ask which base their system used. Half the time the answer is a legitimate carve-out you did not know about, and you have learned something about your own agreement.
Bring the pattern, not the penny. One invoice off by $16 is a rounding argument. Five months of invoices all computing the discount on a base that excludes something your agreement says it should include is a system setting, and reps can get system settings changed.
Do it fresh. Recent invoices get corrected; ones from two quarters ago get explanations. Tolerance rules in AP systems commonly pass anything within 2 to 3 percent of the PO automatically, which is precisely the band a mis-based prompt-pay discount lives in. It will not trip anyone’s threshold. Only a human recomputing it will catch it.
And handle the credit side properly. A published institutional credit memo policy shows what happens on the other end of a slow credit: balances uncollected past 45 days trigger vendor contact, and past 90 days go to collections. Your distributor’s AR department works the same way. A short-paid invoice you never explained becomes a collections item eventually, so never short-pay silently, even when you are right.
Get the terms improved while you are in there
Prompt-pay terms are negotiable, and they are one of the least contested things on the table because they cost the distributor cash flow rather than margin. Worth asking:
- Can the base be defined in writing, including the carve-out list?
- Can the clock start at delivery rather than invoice date, given how invoices actually reach you?
- Can freight be excluded and shown as its own line on every invoice?
- Can a disputed line be carved out of a statement discount rather than voiding the whole statement’s discount?
- Is there a better rate at a shorter window, or on statement terms versus invoice terms?
None of that touches your unit prices, which makes it easier to agree to. If you are heading into a broader pricing conversation anyway, terms belong on the agenda alongside the multipliers: see negotiating with your supply house for the full agenda, and whether to add a second source supplier for how terms compare across two houses.
The short version
- 2/10 net 30 means 2 percent off the discountable merchandise subtotal if paid in 10 days, full amount at 30. Tax and freight are normally outside the base.
- Paying a flat 98 percent of the invoice total takes more discount than you earned and leaves an open balance.
- A supply house computing on too narrow a base shorts you, and it will never show as a visible line.
- Forgoing 2/10 is expensive on an annualized basis, so the real question is whether the discount you took was calculated right.
- Watch tax treatment, freight bundling, non-discountable carve-outs, applied credits, statement versus invoice clocks, and duplicates.
- Recompute five invoices by hand each month. Bring the pattern to your rep, not the penny.
- Never short-pay silently. Ask for a corrected invoice or a credit memo instead.