XRPL AMM Auction Slot Math: When a Trading Fee Discount Pays Off

By XORA · Published 10 August 2026

TL;DR: An XRPL AMM auction slot cuts the holder's trading fee to one tenth of the normal fee, so approximate gross savings equal 90% of the normal fee multiplied by eligible AMM volume. For an empty or expired slot, the official minimum bid formula implies a striking illustrative threshold: fee savings cover the minimum bid when planned volume reaches about 4.44% of pool value, regardless of the chosen normal fee. That result is not a live pool quote and fails if slippage, order book routing, LP token acquisition, an early outbid, or other costs are material.

What the auction slot changes

An XRP Ledger AMM has one auction slot. According to the official XRPL AMM documentation, an LP token holder can bid for it, receive a discounted trading fee for up to 24 hours, and authorize as many as four additional accounts to use the same discount. The slot ends when 24 hours pass or another eligible bidder wins it.

The normal AMM fee is set by liquidity provider voting. XRPL permits a fee from 0% through 1%, in 0.001% increments, and the auction holder pays one tenth of the current fee. Thus, a 0.30% normal fee becomes 0.03%, while a 1.00% normal fee becomes 0.10%. If the normal fee is zero, the AMMBid specification notes that the slot has no immediate fee benefit.

Winning is not free. The bidder pays with that AMM's LP tokens. The bid reduces outstanding LP tokens and thereby increases the proportional ownership of remaining LP tokens. Economically, the trader should bid only when the avoided trading fees are worth more than the LP tokens surrendered plus every cost required to obtain and use them.

Fee savings at four example fee rates

For many small trades, a useful first pass is to multiply qualifying volume by the difference between the normal and discounted rates. Let F be the normal fee as a decimal and V the value of trades that actually execute against the AMM while the discount is active:

Discounted fee = F ÷ 10
Approximate gross savings = V × (F − F ÷ 10) = 0.9 × F × V

This linear estimate is screening math, not an execution quote. XRPL's own auction slot tutorial calculates the same swap twice using the full and discounted fee, then treats the input difference as maximum potential savings. That exact approach captures pool size, trade direction, and slippage. It also warns that a real trade can route partly through order book offers, so total pair volume is not necessarily eligible AMM volume.

Why the empty slot threshold is 4.44% of pool value

The official empty, expired, or final interval minimum bid is M = L × F ÷ 25, where M is the bid in LP tokens, L is total outstanding LP tokens, and F is the normal fee as a decimal. Because LP tokens represent proportional pool ownership, an approximate economic value for that minimum bid is pool value multiplied by F divided by 25.

Set that bid value equal to the simplified fee savings. The normal fee appears on both sides and cancels:

Pool value × F ÷ 25 = 0.9 × F × V
V = Pool value ÷ 22.5 = 4.444% of pool value

For the illustrative $2 million pool, the full slot gross threshold is about $88,889 of qualifying AMM volume. At 0.10%, the minimum bid is worth about $80 and the savings at that volume are $80. At 1.00%, both rise to $800. The fee independence is real within this algebra, but only because both minimum bid and gross savings scale linearly with F.

This is a narrow result, not a universal trading rule. It assumes the slot is available at M, the bidder already holds LP tokens, the LP token redemption value is proportional to pool value, all planned volume executes against the AMM, the trader keeps the slot for the necessary trades, and costs are zero. If the trader first makes a single asset deposit to obtain LP tokens, that deposit can have its own fee and it increases L, which raises the auction price. The official tutorial uses a separate inversion formula for exactly that reason.

The maximum rational LP token bid

A bidder should work backward from savings, not forward from the current bid. Assume the $2 million pool has 100,000 LP tokens outstanding, making the illustrative redemption value $20 per LP token. At a 0.30% normal fee, approximate savings are 0.27% of eligible volume. The gross bid ceiling is savings divided by $20.

Gross LP token bid ceiling = (0.9 × F × V) ÷ LP token value
Prudent ceiling = (savings − all costs − safety reserve) ÷ LP token value

Occupied slots change the answer

An occupied slot is not priced at M alone. The protocol formula is P = B × 1.05 × (1 − t⁶⁰) + M, where B is the previous bid and t is elapsed slot time rounded down to a 0.05 step. The first interval costs B times 1.05 plus M; the final twentieth interval costs only M. Each interval is 72 minutes, making 20 intervals over 24 hours.

Suppose B was 50 LP tokens and M is 12. The outbid price is 64.50 LP in the first interval, still about 64.50 at 50% elapsed, about 64.41 at 90%, about 62.08 at 95%, then 12 in the final interval. This unusual schedule follows the t to the sixtieth power term. It keeps the replacement cost high for most of the day before the last interval resets it to M.

The displaced holder receives a refund of B × (1 − t), except that the refund is zero in the final interval. A new bidder should still compare the full LP tokens debited with its own remaining fee savings. Expected refunds matter to the current holder, not as a discount promised to the new bidder.

Risks the spreadsheet cannot remove

• Early outbid risk: another LP token holder can replace you before planned volume executes. The 24 hour figure is a maximum, not a guaranteed holding period.
• Execution risk: pool balances, the voted fee, order book offers, and slippage can change between calculation and validation. Use the validated or current amm_info fields, then simulate the intended trades.
• LP token acquisition risk: buying or minting LP tokens has price, fee, reserve, and liquidity consequences. The pool's L total can change, moving M.
• Asset risk: XRP and the paired token can move sharply. Issued assets add issuer, freeze, clawback, transfer fee, liquidity, and peg risks depending on their settings.
• Opportunity and tax risk: spending LP tokens gives up a pool claim. Tax treatment of the bid, trades, and LP position depends on jurisdiction and can change the net result.

For the separate question of whether AMM fees overcome pool rebalancing, see our XRP AMM impermanent loss math. For a wider map of native liquidity and other structures, read the XRPL DeFi ecosystem guide and ways to earn yield on XRP. For an entirely different compounding lens, use the exact XRP yield doubling time calculations.

Put your XRP to work with the tradeoffs clear

Auction slots are for traders prepared to model exact swaps and hold LP tokens. They are not fixed income products, and even correct protocol math cannot guarantee a profit. XRP, LP tokens, and issued assets can lose value, and crypto losses can be total.

If managing auction bids is not your preferred route, XORA offers up to 22% APY value (15% native XRP yield, treasury-subsidised during a disclosed bootstrap, plus estimated XORA reward value). It is never guaranteed, reward value can change, and custody and market risks remain.

Put your XRP to work for up to 22% instead of leaving it idle on an exchange. Review the security and custody model, model outcomes with the XRP yield calculator, and start only with an amount appropriate for your risk tolerance.