Fee Estimation

On Bitcoin you bid for block space, and miners fill the roughly ten-minute proof-of-work block with the transactions paying the most per byte. Estimate too low and your transaction lingers in the mempool for hours or stalls until you fee-bump it; estimate too high and you simply burn BTC that could have stayed with the recipient or as change. Good fee estimation finds the smallest fee that still clears within your target window. This guide is for wallet, exchange, and payment developers who construct transactions over the TheRPC endpoint at https://bitcoin.therpc.io/YOUR_API_KEY. It covers what estimatesmartfee returns, how to read the mempool floor as a fallback, how to convert fee-rate units correctly, and how to expose fast/normal/slow tiers to users.

Understanding Fees

• The fee is implicit, not a field. A Bitcoin transaction has no fee parameter — the fee is whatever you leave behind, the difference between the total BTC in the inputs and the total BTC in the outputs. Miners claim that remainder, so under-funding the outputs by too much overpays and over-funding leaves nothing for the miner.
• Fees are priced per virtual byte (sat/vB). Block space is the scarce resource, so what matters is the rate per byte of transaction weight, not the BTC value moved. A larger transaction — more inputs, more witnesses — costs more total fee at the same rate, which is why coin selection affects cost.
• estimatesmartfee returns a feerate in BTC/kvB for a given confirmation target measured in blocks. Ask for confirmation within 1 block for urgency or 24 blocks to save money; the node bases the figure on recent block inclusion.
• The mempool minimum relay fee is a hard floor. getmempoolinfo exposes mempoolminfee: any transaction below it will not even relay across the network. Use it as a fallback when estimatesmartfee has too little data to answer, so you never broadcast a transaction that peers silently drop.

Estimating a Fee Rate

1// Estimate a fee rate for confirmation within N blocks
2const estimateFeeRate = async (confTarget = 6) => {
3  const result = await rpc('estimatesmartfee', [confTarget]);
4 
5  if (!result.feerate) {
6    // Not enough data — fall back to the mempool floor
7    const info = await rpc('getmempoolinfo');
8    return Math.ceil(info.mempoolminfee * 1e5); // BTC/kvB -> sat/vB
9  }
10 
11  // estimatesmartfee returns BTC/kvB; multiply by 1e5 for sat/vB
12  return Math.ceil(result.feerate * 1e5);
13};
14 
15// Offer fast / normal / slow tiers
16const tiers = {
17  fast: await estimateFeeRate(1),
18  normal: await estimateFeeRate(6),
19  slow: await estimateFeeRate(24),
20};

Best Practices

• Handle the no-data case. When the node lacks enough history for your target, estimatesmartfee returns an errors array and no feerate. Never read feerate blindly — check it exists, and fall back to the mempoolminfee floor from getmempoolinfo as the example does, so estimation never crashes during quiet periods.
• Convert BTC/kvB to sat/vB carefully. One BTC/kvB equals 100,000 sat/vB, so multiply the feerate by 1e5. Because BTC values arrive as floating-point, round with Math.ceil after the multiply rather than truncating, and keep all internal math in integer satoshis to avoid rounding drift.
• Offer multiple tiers from several conf_target calls. Calling with targets like 1, 6, and 24 blocks gives natural fast/normal/slow options. Let users trade urgency for cost instead of forcing one fixed rate.
• Prefer opt-in RBF over guessing high. Rather than padding the fee up front to cover worst-case congestion, mark transactions replaceable (BIP-125) and start at a normal tier. If it stalls you can bump the fee on the same transaction — cheaper on average than overpaying every time.
• Size a congestion buffer from live mempool data. During fee spikes, read the current mempoolminfee and recent block fee percentiles (via getblockstats) and add a margin above the estimate so your transaction clears the rising floor instead of falling just under it.

See also

• estimatesmartfee - Estimate a fee rate for a target confirmation window
• getmempoolinfo - Read the mempool minimum relay fee
• getblockstats - Read per-block fee percentiles