Why adding two fee percentages is only an approximation
The buy fee applies to the purchase price and the sell fee applies to the selling price. Because the amounts differ, simply adding the two percentages does not give the exact required spread. Fixed transfer or rebalancing costs also depend on how much BTC you trade.
With buy price B, buy fee b, sell fee s, quantity q and fixed costs C, the break-even selling price is [B × (1 + b) + C ÷ q] ÷ (1 − s). The required spread percentage is the resulting sell price divided by B, minus one, multiplied by 100.
Understand the effect of quantity
A fixed cost is spread across the entire quantity. Doubling the trade size halves its per-BTC contribution to the threshold, assuming the fixed cost stays unchanged. The proportional fees do not shrink in the same way unless your actual fee tier changes.
Larger orders can create additional slippage. A lower calculated threshold at a larger quantity is therefore not proof that scaling improves the trade. Recalculate the buy and sell execution prices using the depth available for that quantity before relying on the comparison.
Use the threshold as a rejection filter
Start with your actual fee tier and include costs needed to complete or rebalance the route. Compare the break-even sell price with an executable bid on the destination exchange. A bid below the threshold fails the cost test before other operational considerations are examined.
A bid above the threshold still leaves execution timing, partial fills and changes in liquidity unresolved. This model assumes identical base quantities and fees charged in the quote currency. It does not include a prediction of how long a price discrepancy will remain available.
Questions about this tool
Why does a fixed withdrawal fee matter more on a small arbitrage trade?
The same fixed cost is divided by fewer BTC. Its contribution to the required sell price rises as the quantity falls, so a route can be uneconomic at one size and pass the cost test at another.
Is the break-even spread my minimum acceptable profit target?
No. Break-even covers the costs entered and leaves zero modeled profit. A profit target, an uncertainty allowance and any additional operational costs require a selling price above that threshold.