By design, Bitcoin's issuance schedule approaches zero around 2140. At that point, transaction fees have to cover the entire cost of mining security — there's no other funding source built into the protocol. Today, fees still make up only a small single-digit share of total miner revenue (issuance + fees) in most periods, which is exactly what makes the long-run question worth tracking now.
Why security has a budget at all
Bitcoin's security model relies on mining being expensive enough that attacking the network (accumulating enough hash power to rewrite history) costs more than any plausible gain from doing so. That cost is funded entirely by what miners get paid: the block subsidy (newly issued bitcoin) plus transaction fees.
This site reconstructs it from free Coin Metrics fields as IssTotUSD + FeeTotNtv × PriceUSD, the same underlying calculation used for the Puell Multiple.
The long-term question this metric is really about
The block subsidy halves roughly every four years and is scheduled to reach zero around 2140, per Bitcoin's fixed issuance schedule. At that point, by design, 100% of miner revenue — and therefore 100% of network security spending — has to come from transaction fees. Today, fees typically make up a small single-digit percentage of total miner revenue in most periods, spiking higher only during periods of unusually high transaction demand (like fee-market congestion events).
What a low fee share implies, and doesn't
A persistently low fee share doesn't threaten security today — issuance still funds the overwhelming majority of the budget for the foreseeable future, and price appreciation has historically outpaced the halving of the subsidy in USD terms across every completed cycle so far. The long-run question is whether transaction fee demand grows enough over the coming century to replace issuance as the dominant security funder before it disappears — something no current data can settle, since it depends on demand for Bitcoin block space decades from now.