Track usage against a monthly data cap.
Tracking usage against a cap needs two figures rather than one: how much allowance is left, and what daily rate that permits over the days remaining. Projecting forward from the rate so far shows whether you are on course to exceed the cap while there is still time to act. A projection above 100 percent means the current pace will breach the cap before the cycle resets.
Cap tracking and projection
Remaining = cap - used; safe daily = remaining / days left; pace = used / day number; projected = pace x cycle length
It depends on the provider. Some charge per additional gigabyte or per block, some throttle the connection to a low speed for the rest of the cycle, and some do neither and simply send a warning. Check which applies before relying on overage.
Providers often project from a rolling seven-day average rather than the whole cycle to date. If your usage is uneven — a large download early in the month — the two methods diverge, and the whole-cycle figure is the more conservative.
On most fixed-line and mobile plans, yes — the cap is combined upstream and downstream traffic. That matters if you run cloud backup or upload video, where upstream can be the larger half.