28-Day Billing Cycles Explained: Why You Pay 13 Times a Year
A 28-day billing cycle means your card is charged every four weeks instead of once per calendar month. Because a year has 365 days and 365 ÷ 28 is just over 13, you end up paying thirteen times a year rather than twelve, so a plan that looks "monthly" costs about 8.6% more per year than the sticker suggests. In the AI generator space, Promptchan is the clearest example: its official plans renew every 28 days. This article explains the arithmetic, shows why your renewal date keeps moving, and gives you a simple way to compare 28-day plans fairly with calendar-month plans and one-time credit packs. All services discussed are for adults 18+ only.
What "billed every 28 days" actually means
A calendar-month subscription renews on the same date each month: start on the 15th, pay on the 15th. A 28-day subscription renews exactly 28 days after the previous charge, whatever the calendar says. Four weeks is shorter than every month except a non-leap February, so each renewal arrives a few days earlier in the month than the one before.
Nothing else about the plan has to be different. You still get a fixed allowance per period, the plan still renews automatically, and cancelling usually stops the next renewal rather than the current period. The only change is the length of the period, and that one change quietly adds a thirteenth payment to every year.
Where you will see it
Four-week cycles are common in gym memberships, some phone plans and a handful of software services. On this site, the tool that uses it is Promptchan: its official pricing lists Plus at $14.99, Premium at $23.99 and Pro at $33.99, each billed every 28 days, with 100, 400 and 800 Gems per cycle, and Pro adding unlimited casual generations, video, voice and 4K. The full breakdown is on our Promptchan pricing page, and our Promptchan review covers the product itself. Prices can change, so confirm them at checkout.
The thirteen-payment arithmetic
Here is the yearly cost of each Promptchan plan if you never cancel, compared with what the same price would cost on a calendar-month cycle. The figures are plain multiplication of the listed prices.
| Plan | Price per cycle | 13 cycles (28-day) | 12 cycles (monthly) | Monthly equivalent |
|---|---|---|---|---|
| Plus | $14.99 | $194.87 | $179.88 | ≈ $16.28 |
| Premium | $23.99 | $311.87 | $287.88 | ≈ $26.06 |
| Pro | $33.99 | $441.87 | $407.88 | ≈ $36.92 |
The "monthly equivalent" column is the price multiplied by 365 ÷ 28 and divided by 12. It is the number to use when you compare a 28-day plan with a competitor that bills per calendar month. Promptchan Pro, at $33.99 per cycle, behaves like a plan of about $36.92 a month.
The quick rule of thumb
To convert any 28-day price into a fair monthly figure, multiply it by about 1.086. To get the yearly cost, multiply it by 13. That is all the maths you need, and it works for any service that bills every four weeks.
Is this a hidden fee?
No. The cycle length is stated, and the price per cycle is what you pay. The issue is only one of perception: people mentally file "$33.99" as "$33.99 a month" and budget for twelve charges. The thirteenth one is not a surprise fee, it is a payment you did not count. Treat it as part of the price.
Why your renewal date keeps moving
Because every renewal is exactly 28 days after the last one, the date drifts backwards through the month. That matters if you match subscriptions to your payday or try to remember a single "renewal day".
Take an example start date of 15 March 2027. The charges would fall on 15 March, 12 April, 10 May, 7 June, 5 July, 2 August, 30 August, 27 September, 25 October, 22 November, 20 December, 17 January and 14 February, with the next one on 13 March 2028. Two observations:
- One calendar month gets two charges. In this example August is charged twice, on the 2nd and the 30th. If you set a monthly budget, that month will look like it went over.
- The calendar year can hold 14 charges. Start on 1 January 2027 and the 13th renewal lands on 31 December, so that calendar year contains fourteen payments. Over any 365-day stretch it averages out to about 13.
The practical consequence is simple: a recurring monthly reminder in your calendar will not match a 28-day plan. Set the reminder every four weeks instead, or check the next renewal date in your account each time you pay.
How 28-day cycles interact with allowances
Credit-based tools refresh their allowance when the cycle renews. On Promptchan, the Gem allowance refreshes with each 28-day period. A shorter period is not bad in itself: you get thirteen refreshes a year instead of twelve, so per refresh you are paying the listed price for the listed allowance. The fair comparison is per day, not per period.
Where it can matter is unused allowance. Whether leftover Gems carry over into the next cycle is not something we could confirm from public pages, so treat rollover as [unverified] and check inside your account. If they do not roll over, a shorter cycle gives you slightly less time to use each batch. Our explainer on why AI porn generators use tokens goes into how allowances and per-action costs fit together, and the AI porn generator pricing guide explains the main billing models side by side.
Cost per image still depends on use
The cycle length changes the yearly total, but your real cost per image depends far more on how much you actually generate. If you only open the app once or twice per cycle, the thirteenth payment is the least of your problems. Our cost per image calculation shows how usage changes the picture on Promptchan Pro and other tools.
Comparing a 28-day plan with other billing models
When you shop around, line up three types of pricing on the same basis: cost per year at your expected usage.
Calendar-month subscriptions
These renew on the same date each month and charge twelve times a year. For example, OurDream AI's FAQ lists $19.99 a month on the monthly plan, or $9.99 a month on a yearly plan paid as $119.88 upfront. To compare it with Promptchan Plus, use Plus's monthly equivalent of about $16.28, not $14.99. Our Candy AI vs OurDream AI comparison covers OurDream's plans in more detail.
Annual plans
Annual plans charge once and avoid the cycle-length question entirely, but they lock you in. Whether they pay off depends on how sure you are that you will still use the tool in month ten. We weighed that in are AI porn generator annual plans worth it.
One-time credit packs
Pay-as-you-go tools have no cycle at all. GeneratePorn.ai sells coin packs that, per its homepage, do not expire, and nothing renews, so there is no thirteenth payment to plan for. The trade-off is that you top up manually. Our GeneratePorn.ai vs Promptchan comparison puts the two models head to head, and the GeneratePorn.ai review covers the product. More options without recurring billing are in our list of no-subscription AI porn generators.
A five-step routine for 28-day subscriptions
- Convert the price. Multiply the cycle price by 13 for the yearly cost and by about 1.086 for a monthly equivalent before you compare.
- Note the first renewal date. It is 28 days after your first payment, not the same date next month.
- Set a four-weekly reminder. Put it a few days before each renewal so you can decide whether to keep going.
- Budget for the double-charge month. Once a year, a calendar month will contain two payments. Knowing which one avoids a surprise.
- Cap the card if you want certainty. A virtual card with a spending limit stops a renewal you forgot about. We explain how in prepaid and virtual cards for adult AI subscriptions.
If you decide to stop, cancel before the next 28-day mark. The steps for the main tools are in our guide on how to cancel an AI porn generator subscription, and refund rules are compared in AI porn generator refund policies compared.
The bottom line
A 28-day billing cycle is not a trick, but it is easy to misread. You pay thirteen times a year, the renewal date slides earlier every month, and one calendar month each year carries two charges. On Promptchan's listed prices, that turns Pro's $33.99 into about $36.92 a month and $441.87 a year. Convert prices before you compare, set reminders every four weeks rather than monthly, and if recurring billing does not suit you, look at one-time credit packs instead.