Cell phone plan calculator

Gigabytes used cannot be negative: that case fails with invalid_input and is not billed.

How base fee, per-gigabyte rate, and usage become a monthly total

Most consumer wireless plans still bill a monthly identity you can write on one line: total_cost equals base_fee plus per_gb times gb_used. This endpoint does not look up carrier catalogs, does not convert currencies, does not split a family plan across lines, and does not model taxes, regulatory fees, device installments, or unlimited buckets that stop metering after a fair-use cap. Unlimited marketing that is actually a zero per-gigabyte rate is allowed and then data_cost is zero and the total equals the base fee.

Input contract, response shape, and error behaviour

POST a JSON body with three required finite numbers. base_fee is the monthly access charge in one currency unit; aliases base, monthly_fee, monthly_base, plan_fee, and fee map to the same field so a bill export can keep its column names. per_gb is the price of one gigabyte in that same currency; aliases rate, per_gigabyte, gb_rate, rate_per_gb, and per_gb_rate are accepted. gb_used is the gigabytes counted in the cycle; aliases gigabytes, gigabytes_used, data_gb, usage_gb, and gb are accepted. Each numeric field may arrive as a JSON number or a numeric string. Missing fields, empty strings, and non-finite values fail with invalid_input naming the field. Zero usage is valid and returns total_cost equal to the base fee. On success the body echoes base_fee and per_gb rounded to two decimal places and gb_used, then reports data_cost as the usage term rounded to two decimal places, total_cost as the monthly sum, and a formula string restating the identity. Example: forty of base fee, ten per gigabyte, and five gigabytes yields data_cost of fifty and total_cost of ninety. Pair the JSON with your own rule for taxes, roaming, and family-line splits, because this endpoint prices one line against one rate; it does not assign a carrier, apply a fair-use cap, or convert currencies. Keep requests small, pin the rounded total_cost in tests, and you get a reproducible monthly figure for worksheets, LMS plugins, and edge workers without standing up a billing engine of your own.

Where a cell phone plan calculator earns its keep

Households comparing a forty-plus-ten overage plan against a seventy unlimited quote use a cell phone plan calculator to turn last month's gigabytes into one comparable dollar figure before they switch. A cell phone plan prices one access fee against one metered data rate and returns money. Failed validation does not consume balance. Taxes, device installments, roaming, and family-plan splits stay outside this identity on purpose: fold them in your own layer after you have the raw plan dollars. Budgeters drop a planned base fee and the published per-gigabyte rate, then pin total_cost so the rest of the month's envelope already reflects the wireless line. Teachers and exam-prep generators need a deterministic oracle for the classic word problem — a cell phone plan costs a dollars plus b per gigabyte; if you use g gigabytes, what is the monthly cost — that marks negative usage as invalid_input instead of silently returning a negative bill a buggy grader might treat as success. Product teams wire the same contract into offer pages and onboarding quizzes that must show total_cost for a sample usage without calling a carrier API. This is a different search and a different job from an annual subscription roll-up, from a cash-back reward, and from a phone-carrier lookup: those tools normalize billing cycles across a list of services, multiply spend by a reward percent, or resolve a number to a network. Keep gigabytes used non-negative, pin the rounded total_cost in tests, and you get a reproducible monthly amount for teasers, bill checks, and edge workers without standing up a wireless ledger of your own.