Compare wifi and cellular data usage.
Total data usage says nothing about cost, because only the cellular portion is usually metered. Splitting the total by wifi share isolates the part that counts against a mobile allowance, and comparing that with the plan size shows the overage directly. Shifting even a few percent more of the total onto wifi often eliminates an overage entirely, which is why the split matters more than the total.
Wifi and cellular split with overage
Wifi GB = total x wifi share; cellular GB = total - wifi GB; overage GB = max(0, cellular GB - plan); cost = overage GB x rate
Both iOS and Android report cellular data per cycle, and most routers or mesh apps report total household traffic per device. Dividing the cellular figure by the device's total gives the split directly.
The overage figure is the answer in gigabytes. Divide it by the total to see the percentage point shift needed — often just three or four points, achievable by restricting video and backup to wifi.
No. Wifi calling routes voice over the wifi connection, so it consumes home or public broadband rather than cellular data, and it normally draws on your call allowance rather than your data allowance.