"Add 10% for the buyer's premium" is close enough for a rough guess and wrong in real money on an actual bid. Equipment auctions stack up to four separate charges on top of the hammer price — a capped premium, an online-bidding premium, a card surcharge, and tax on more than just the hammer price — and a flat mental percentage cannot represent a cap, a conditional fee, or a compounding tax base at the same time.
| Charge | Flat-percentage mistake | What this calculator does |
|---|---|---|
| Buyer's premium cap | Keeps applying the percentage past the point where a per-lot cap has already stopped the real charge, overstating the cost on an expensive lot. | Charges the lesser of the percentage or the cap you enter, and says on the page when the cap applied. |
| Online-bidding premium | Forgets it entirely, or double-counts it as part of the "buyer's premium" percentage, when it is usually a separate, stacking fee. | Adds it only when you say you're bidding online, on top of the buyer's premium, never merged into one number. |
| Card surcharge | Applied to the hammer price alone, if applied at all — when it is normally charged on the whole invoice including every premium and tax. | Applies the surcharge to the full invoice total, only when you say you're paying by card. |
| Sales tax base | Taxes the hammer price only, understating the bill wherever the premium is taxed too — or assumes tax applies when the purchase is actually exempt. | Taxes the full purchase price (hammer price plus every premium) unless you mark the purchase exempt, and always discloses which assumption it used. |
| Is anything I enter uploaded anywhere? | N/A | No. The calculator runs entirely in your browser; nothing is sent to a server. |
Because many equipment auctions cap the buyer's premium per lot — commonly a few thousand dollars — so a flat 10% in your head keeps growing past the point where the cap has already stopped the real bill. On a $45,000 tractor with a $2,500 cap, mentally adding 10% ($4,500) overstates the premium by $2,000.
Because the buyer's premium and the online-bidding (internet) premium are two separate, stacking fees at most houses that run a live online platform. A mental estimate of just the buyer's premium misses the extra 2-3% charged only to bidders using the platform, which a bidder on the floor in person would not owe.
Not unless you remember to add it separately, and it applies to the whole invoice — hammer price, every premium, and tax — not the hammer price alone. Paying by ACH or check instead of a card avoids this fee at houses that charge one.
Under most published auction terms, on the full purchase price — hammer price plus every premium — which a mental estimate that taxes only the hammer price will understate. Some houses or states tax the hammer price alone, and some equipment purchases are exempt entirely, so the specific auction's printed terms are what actually governs, not a general rule.
No — you supply the buyer's premium rate and cap, the online and card fees, and the tax rate from the auction's own printed terms. What it removes is the arithmetic mistake of adding them up in the wrong order or forgetting one of them, not the need to read the terms in the first place.