Haflagah: How to Calculate the Interval Veses
6 min read · October 22, 2026
The veses haflagah anticipates that the next period will come after the same interval as the last one. It is the most arithmetic of the vestos, and the one where customs differ most.
Measuring the interval
In the common reckoning, count from the day the previous period began to the day this one began, counting both days. If that is 27, the haflagah falls on day 27 counting from this period's onset as day one, at the onah this period began in.
Days or onos
Some count the interval in onos — half-days — rather than in days (the Chazon Ish). A period at night after one by day then gives a different answer from counting days. Your rav decides which you follow.
From the onset or from the hefsek
Most count from onset to onset. Some, including Chabad, count from the hefsek taharah. Under that reckoning the interval and the next haflagah are both measured from the hefsek.
Shorter and longer intervals
When a shorter interval follows a longer one, many keep the longer haflagah as well until it has passed without a period; others let the shorter one replace it, at once or after it repeats. A calendar should keep whichever your custom says.
Fixed by repetition
Three equal intervals — which takes four periods — make a veses haflagah kavua. A steady change, such as 31, then 32, then 33 days, can also form a pattern (a dilug); these are questions to bring to your rav.
This guide explains the general rules and the common customs. Where poskim differ it says so — it does not rule. For your own questions, ask your rav.
Questions
- How do I count the haflagah?
- Count from the onset of the previous period to the onset of this one, counting both days; the haflagah falls that many days after this onset, counting it as day one.
- Is the haflagah counted from the onset or the hefsek?
- Most count from the onset. Some, including Chabad, count from the hefsek taharah.