Bitcoin Forum

Is there an upward limit per sendmany transaction? 100, 255, 1000?

Any way to easily calculate the appropriate transaction fee for such a transaction?

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See GetMinFee() in main.h for the code which determines the minimum transaction fee.

The part you're describing is implemented as follows:

// Raise the price as the block approaches full
if (nBlockSize != 1 && nNewBlockSize >= MAX_BLOCK_SIZE_GEN/2)
{
    if (nNewBlockSize >= MAX_BLOCK_SIZE_GEN)
        return MAX_MONEY;
    nMinFee *= MAX_BLOCK_SIZE_GEN / (MAX_BLOCK_SIZE_GEN - nNewBlockSize);
}
where:

nBlockSize depends on who we are:
if we're a user making a new transaction, nBlockSize is 1
if we're a node deciding whether to relay an incoming transaction, nBlockSize is 1000
if we're a miner deciding which transactions to include in a new block, nBlockSize is the size of the block we're building before adding the new transaction
nNewBlockSize is the size of the block including the new transaction
MAX_BLOCK_SIZE_GEN is 500kB
MAX_MONEY is 21 million BTC (all the money in the world)
so it's saying:

if we're either deciding whether to relay or mine the transaction, and adding the transaction would make the block over 50% full:
never allow a transaction that makes the block 100% full
otherwise scale the fee up by a factor of 500kB / (500kB - nNewBlockSize)
Note: by rearranging the symbols, we get exactly what Meni wrote in his answer:

Fee = Normal fee / ((500kB - size) / 500kB) = Normal fee / (1 - size / 500kB)