What a phone plan actually costs
The advertised price is the service charge, and it is usually the smallest of the four things on your bill. The other three are taxes and regulatory surcharges, device installments, and promotional credits, and each behaves differently enough that no single monthly number describes the plan.
Taxes and surcharges apply to the service portion on postpaid accounts and are quoted separately, so an advertised $180 becomes something closer to $200. Prepaid brands normally quote a tax-inclusive price instead, which is why a prepaid plan that looks slightly cheaper on paper is often substantially cheaper in practice.
Device installments are a loan, not a service charge. Financing an $800 handset over 24 months adds $33.34 a month that has nothing to do with connectivity, is not taxed as a telecom service, and does not disappear if you switch carriers — the balance becomes due.
Promotional credits are the part that makes comparison hard. A trade-in offer worth $800 is almost never paid up front; it arrives as $33.34 a month for 24 months, conditional on keeping that line active on a qualifying plan for the whole term. Leave early and the remaining credits vanish while the device balance does not.
This calculator holds all four apart, so you can see the monthly cost today, the monthly cost after the promotion ends, and the total over whatever period you choose.
The formula, term by term
Start with the service charge S for the whole account at the tier your line count qualifies for. Multi-line pricing is steeply non-linear — the fourth line is usually far cheaper than the first — so entering a whole-account price rather than a per-line price avoids the most common modelling error.
Subtract the autopay discount a·n. It is quoted per line, applies monthly, and on most US carriers requires payment from a bank account rather than a credit card, which is a real condition and not a formality. The result is the charge that taxes are assessed on.
Multiply by 1 + t. The surcharge percentage varies widely by state and locality because it bundles federal Universal Service Fund contributions, state universal service and 911 fees, and carrier cost-recovery charges. The reliable way to get your own t is arithmetic on a real bill: divide the total of the taxes-and-fees section by the plan charge above it.
Add device installments D. They are untaxed as service, run for a fixed number of months, and are the reason two plans with identical service pricing can differ by hundreds of dollars over two years.
Subtract credits C. These are applied to the bill after tax, which is why a $16.67 credit reduces your bill by exactly $16.67 rather than by a tax-adjusted amount.
The total is then two rectangles: the promotional monthly figure for min(N, k) months, plus the post-promotional figure for whatever remains. Whether the bill steps up or down at month k+1 depends on which is larger, D or C — the calculator reports the direction rather than assuming it.
Worked example: four lines, postpaid with a trade-in, against prepaid
A family of four is offered a postpaid plan at $180 a month for all four lines, with a $5 per line autopay discount, taxes and fees running 12%, two financed phones at $33.34 a month combined, and a trade-in promotion worth $16.67 a month for 24 months. The alternative is a prepaid brand at $100 a month all-in for four lines, bringing their existing phones.
- Plan A service after autopay. $180.00 − 4 × $5.00 = $160.00.
- Plan A taxes and fees. $160.00 × 12% = $19.20.
- Plan A monthly during the promotion. $160.00 + $19.20 + $33.34 − $16.67 = $195.87.
- Plan A monthly after month 24. $160.00 + $19.20 = $179.20. The installments and the credits both stop, and because the installments are worth more per month than the credits, the bill falls by $16.67.
- Plan B monthly. $100.00 with no separate surcharges and no installments = $100.00.
- Totals over 24 months. Plan A: 24 × $195.87 = $4,700.88. Plan B: 24 × $100.00 = $2,400.00.
- Difference. $4,700.88 − $2,400.00 = $2,300.88 more for Plan A over two years, or $95.87 a month.
That difference is the real decision, and it buys two financed handsets worth about $800 together. Netting those off, the family is paying roughly $1,500 over two years — about $62 a month — for the postpaid network, priority data and support. Whether that is worth it is a judgement; the point is that it is now a judgement about $62 rather than about a $180 sticker.
Extend the comparison to 36 months and the gap widens: Plan A adds 12 months at $179.20 for a total of $6,851.28, Plan B adds 12 at $100.00 for $3,600.00, and the difference grows to $3,251.28.
How to read the comparison
Look at the post-promotional monthly figure before the promotional one. It is the price you will pay for most of the plan's life, and it is the number carriers advertise least. A plan whose monthly cost today is low only because credits are absorbing an installment is not cheap; it is deferred.
The cumulative chart shows whether the plans cross. Two plans cross when the one with the higher monthly cost carries device value that the other does not — the postpaid line starts higher and stays higher, but the gap in month 24 is smaller than the handsets are worth. If the lines never cross, the comparison is simply which plan is cheaper, and the answer does not change with time.
Cost per line per GB is the right metric only when both plans have a finite premium allowance. Once a plan is genuinely unlimited at full speed, dividing by an allowance is meaningless, which is why the calculator drops that row when either allowance is entered as zero. For plans with deprioritisation thresholds, remember the threshold is not a hard cap — you keep working past it at reduced priority during congestion.
Treat any promotional credit as conditional money. Carriers forfeit remaining credits when you pay off the device early, cancel the line, or move it to a plan that does not qualify. The calculator's warning quantifies the total at stake so you can weigh it against the flexibility you are giving up.
What a financed handset adds per month
| Handset price | 24 months | 30 months | 36 months |
|---|---|---|---|
| $400 | $16.67 | $13.33 | $11.11 |
| $600 | $25.00 | $20.00 | $16.67 |
| $800 | $33.33 | $26.67 | $22.22 |
| $1,000 | $41.67 | $33.33 | $27.78 |
| $1,200 | $50.00 | $40.00 | $33.33 |
| $1,600 (two at $800) | $66.67 | $53.33 | $44.44 |
Divide the retail price by the number of months. A longer term lowers the monthly figure without reducing the total, and it lengthens the period over which leaving the carrier triggers an accelerated balance.
Comparison mistakes that cost real money
- Comparing a postpaid pre-tax price with a prepaid all-in price. The surcharge percentage on postpaid service is routinely double digits. Enter it, or you are comparing two different quantities.
- Treating a trade-in as cash. Almost all US trade-in offers pay out as monthly bill credits over the financing term and are forfeited on early exit. The advertised value is the maximum, not the amount.
- Ignoring the month the credits end. If the installments and credits do not stop on the same date, your bill changes twice. Check both end dates on the equipment agreement, not the marketing page.
- Assuming autopay is free. Several carriers pay the full discount only for bank-draft autopay and reduce or withhold it for credit-card autopay, which can quietly cost you the rewards you were banking on.
- Forgetting the device balance on switching. Leaving a carrier accelerates the unpaid installment balance. A switching bonus that covers it is worth its face value; one that does not is worth face value minus what you owe.
- Counting deprioritisation as a cap. A 50 GB premium allowance does not stop working at 50 GB; it drops in priority during congestion. Pricing it as a hard cap overstates the value of a larger allowance.
Where the surcharges on a US wireless bill come from
The taxes-and-fees block on a postpaid bill mixes genuine government charges with carrier-imposed cost recovery. Federal Universal Service Fund contributions are set quarterly by the FCC as a percentage of interstate and international revenue and are passed through to subscribers. State and local charges add universal service, 911, and sometimes utility or gross-receipts taxes. Regulatory and administrative charges are the carrier's own line items and are not taxes at all, though they appear in the same block. Because the mix is jurisdictional, the only reliable figure for your household is the one on your own bill.
Where this sits among the other household subscription tools
A phone plan is one of several recurring services where the advertised price and the paid price diverge, and the same modelling applies to all of them: separate the promotional period from the steady state, and total both. The streaming vs cable cost calculator does the same job for television, where equipment rental and broadcast fees play the role that device installments play here.
If the decision is really about whether to keep a device longer rather than which carrier to use, the arithmetic is a payback question: the monthly installment you avoid against the value of the newer handset. The stacked coupon and double discount calculator handles the related question of what a layered promotion is actually worth once the percentages are composed properly.
Household budgeting for the whole set of recurring bills sits alongside this: the roommate utility bill split calculator divides shared services among housemates, and the credit card cash back calculator values the rewards you might be trading away by moving to bank-draft autopay.
Two limits worth stating. This calculator compares two plans at a time — for three or more, run it twice against a common baseline. And it assumes 0% device financing, which is the standard structure on US carrier equipment installment plans; if you are financing a handset through a third party at interest, model that separately as a loan.