How to work out your maximum bid

If you know the most you can spend in total, you can't just take the fees off your budget. The fees are a percentage of your bid, so you have to work backwards.

The mistake almost everyone makes

Say your budget is £1,000 and the fees are a 20% premium plus a 4% online fee, with VAT on both. It's tempting to bid around £800 and assume the rest covers the fees. But a bid of £800 actually costs £1,030.40, which is £30.40 over budget. The fees grow with every pound you bid.

The method

Take away any fixed costs, such as delivery, from your budget.
Add up the percentages to get a "cost factor" (the formula is below).
Divide what's left by the cost factor.

Maximum bid = (budget − delivery) ÷ (1 + premium + online fee + VAT × (premium + online fee))
Use percentages as decimals, so 20% is 0.20. If VAT also applies to the winning bid, add the VAT rate to the bottom line as well.

Worked example

Budget £1,000, 20% premium, 4% online fee, 20% VAT on the fees, no delivery.

Cost factor = 1 + 0.20 + 0.04 + 0.20 × (0.20 + 0.04) = 1 + 0.24 + 0.048 = 1.288

Maximum bid = £1,000 ÷ 1.288 = £776.40

Check it by adding everything up:

Bid	£776.40
Buyer's premium (20%)	£155.28
Online bidding fee (4%)	£31.06
VAT at 20% on the fees	£37.27
Total (about £1,000, each line is rounded to the penny)	£1,000.01

What changes with delivery or VAT on the bid

Delivery of £50: (£1,000 − £50) ÷ 1.288 = £737.58
VAT also charged on the winning bid: the cost factor rises to 1.488, so £1,000 ÷ 1.488 = £672.04

Delivery is a fixed amount, so it comes off your budget before you divide. Percentages go into the cost factor.

Working backwards from a resale price

If you're buying to sell on, start from what you want to make instead. Your maximum total cost is the resale price minus your selling fees and the profit you want:

Maximum total cost = resale price × (1 − selling fee − target margin)

Say you expect to resell for £1,500, the platform takes 10%, and you want a 20% margin:

£1,500 × (1 − 0.10 − 0.20) = £1,050 maximum total cost. Then divide by the same cost factor: £1,050 ÷ 1.288 = £815.22 maximum bid.

Check it: £1,500 − £150 selling fees − £1,050 cost = £300 profit, which is 20% of £1,500.

One practical tip

Auctioneers take bids in set steps called bid increments, usually listed in the conditions of sale. Round your figure down to the nearest step you can actually bid.

How BidCalc helps

Switch to "I have a budget", enter what you can spend in total, and BidCalc works backwards through every fee to show your maximum bid. If you plan to resell, tick "I plan to resell this", enter the resale price, your selling fees and a target margin, and it shows the highest bid that still hits that margin.

See the £1,000 budget example Open the calculator

General information only, not tax or financial advice. Auction houses set their own fees and terms, so always check the conditions of sale and confirm the details with the auction house before you bid.