Bloom filter calculator
A calculator to find the optimum parameters for a Bloom filter, and to experiment to find the effects of changing them. For background and formulae see our page on Bloom filters.
Examples
Q. I have 6550 items in my set, how big should my filter be and how many hash functions (k) for a 1% probability of a false positive?
INPUT: n=6550, p=0.01
n=6550 m=62783 (7.7 kB) m/n=9.6 k=7 p=0.010039 (1.00%)
A. Use a filter of 7.7 kB (OK, round up to 8.0 kB, 216 bits) and use k=7 hash functions.
Q. I have 6550 items in my set and propose to use a bit filter of length 216 (8.0 kB), what is the optimum k and subsequent probability p of a false positive?
INPUT: n=6550, m=2^16
n=6550 m=65536 (8.0 kB) m/n=10.0 k=7 p=0.008172 (0.82%)
A. Use k=7 hash functions to get a 0.82% probability of a false positive.
Q. I have 6550 items in my set and propose to use a bit filter of length 216 with k=5 hash functions, what is the probability of a false positive?
INPUT: n=6550, m=2^16, k=5
n=6550 m=65536 (8.0 kB) m/n=10.0 k=5 p=0.009411 (0.94%)
A. The probability of a false positive is 0.94%
Q. I have 6550 items in my set, how big should my filter be and how many hash functions (k) for a 5% probability of a false positive?
INPUT: n=6550, p=0.05
n=6550 m=40841 (5.0 kB) m/n=6.2 k=4 p=0.050268 (5.03%)
A. Use a filter of 5.0 kB and use k=4 hash functions.
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This page first published 4 January 2018. Last updated 17 April 2020.