Why the advertised price is the wrong thing to compare
Four separate things end up on a phone bill and only one of them is on the advertisement. The plan charge is what is quoted, usually per line and usually at the headcount that makes it look best. The device payment is a loan, generally spread over twenty-four months, and it is the reason a bill drops sharply on a month you did not expect. Overage or throttling is what happens when the data runs out, and the two are completely different kinds of cost. And taxes, surcharges and regulatory fees make up a block that carriers frequently quote separately from the plan price, so two plans with the same headline number do not necessarily produce the same bill.
Comparing on the plan charge alone gets the ranking wrong often enough that it is worth the ten minutes to fill in all four for each option. The place to get the numbers is a current bill for the plan you have and a written quote or the checkout page total for the ones you do not.
Per-line pricing and why headcount decides everything
Multi-line pricing is not linear. The first line carries most of the cost and each additional line is discounted, so the per-line figure quoted in advertising usually assumes a household of three or four. A plan that is excellent value at four lines can be poor value at one, and moving a line off a family account can raise the per-line cost for everyone left on it.
This calculator asks for the total price at your actual headcount rather than a per-line figure, precisely so that structure cannot hide. When you price an alternative, price it at the number of lines you would actually move, including any line you would leave behind.
Allowances, throttling and what a gigabyte is worth
Past the allowance a plan either charges you or slows you down, and the two need different reasoning. A per-gigabyte charge is a number and goes straight into the total. Throttling is a change in service quality that costs nothing, which is why an unlimited plan with a soft cap can look identical to one without on a spreadsheet and feel very different in use.
The cost-per-gigabyte column is useful for a sanity check and misleading if you lean on it. It divides the entire cost of the plan by the data you happen to use, so light users always look like they are paying enormous rates and heavy users always look efficient. That is arithmetic, not a finding. The number that matters is the total, and cost per gigabyte only earns its place when you are deciding whether to buy a larger allowance you do not currently need.
The taxes and fees block
US wireless bills carry a set of line items below the plan charge: federal and state taxes, a universal service contribution, local charges that depend on the billing address, and carrier-imposed administrative or regulatory recovery fees that are not taxes at all despite the naming. Together they can be a noticeable fraction of the bill.
Two things follow. First, an all-in price that includes taxes and fees is not comparable to a plan price that excludes them, and finding out which you are being quoted is the first question to ask. Second, the amount is address-dependent, so a figure someone else reports for the same plan may not be yours. Add up the lines on your own bill and enter that; if you are pricing a plan you do not have, ask the carrier for a full first-bill estimate rather than a plan price.
Reading the result without kidding yourself
The total over the term is the comparison, and the column showing what each plan costs after the device is paid off is the one that describes the years afterwards. A plan can win the twenty-four months on the strength of a device credit and then be the most expensive thing on the list from month twenty-five onward, which is a fine trade if you intend to move again and a bad one if you do not. Switching also carries friction the numbers do not hold: porting a number, activation windows, whether a device is unlocked and whether a promotional credit survives a plan change. Decide the money question here, then check that list before acting on it.
Questions people ask
Why is my bill higher than the plan price I signed up for?
Almost always some combination of three things. Device payments, which are a separate loan line and continue for a fixed number of months regardless of the plan. Taxes, surcharges and regulatory fees, which many carriers quote separately from the plan price and which depend on your billing address. And a first bill that covers a partial month plus the coming month, sometimes with activation or upgrade charges attached, which makes it larger than every bill that follows. Compare the second or third bill against the quote rather than the first.
Is unlimited data actually unlimited?
It usually means no charge for extra data rather than no limit on it. Most unlimited plans include a threshold of high-speed data, after which speeds may be reduced either always or when the network is busy, and video streaming is frequently capped at a resolution regardless of usage. None of that appears as money on a bill, so it never shows up in a cost comparison, which is exactly why it is worth checking separately. Look for the priority data threshold, the policy on network management, and the hotspot allowance, which is often a much smaller number than the phone allowance.
Should I pay for a phone outright or take the monthly device payment?
Compare the two over the same window with everything included, which is what the one-off cost and device payment fields are for. Carrier device payments are commonly interest-free, so paying monthly costs no more in nominal dollars and keeps the money in your account, which favours the payment plan. What weighs against it is that a device payment agreement usually ties a promotional credit to staying on a particular plan for the full term, so leaving early forfeits the remaining credit and the balance comes due. Buying outright costs more today and leaves you free to switch on any month.
How much data do I actually need?
Check rather than guess: both phone operating systems report cellular data used per billing period in settings, and the carrier app breaks it down by line. Look at the highest of the last three months, not the average, because the month that matters is the one that would have triggered an overage or a throttle. If most of your use is on Wi-Fi at home and work, the cellular figure is often far lower than people expect. If you tether a laptop regularly, check the hotspot allowance separately, because it is usually a smaller number than the phone data allowance and is metered on its own.