Fixed vs Variable APR: What Is APR & What Does “Per Annum” Mean for Interest?
(With clear UK vs US differences where it matters)
APR shows up on every credit ad, card agreement, and mortgage page. It looks like the only number you need. Helpful? Yes. Complete? Not quite.
Here’s the plain-English guide to what APR really is, what “per annum” means in practice, and how to choose between fixed and variable APR—with the UK and US rules separated so nothing gets muddled.
- APR = Annual Percentage Rate: the yearly cost of borrowing. On loans/mortgages, APR blends the rate + certain fees for apples-to-apples comparisons. On cards, APR usually labels the interest rate on transactions (many fees sit outside the APR number).
- Per annum = per year—but card interest typically accrues daily and lands on your monthly statement.
- Fixed APR → predictable payments, protection from rate rises. Variable APR → tracks a benchmark (e.g., base/prime + a margin) and can fall… or jump.
If you carry a balance, your effective yearly cost can exceed the printed APR because of daily compounding.
What Is APR? (Simple, precise)
APR is an annualised snapshot of the cost of borrowing.
- Loans/Mortgages (both regions): APR blends the interest rate with certain lender-required fees so you can compare products more fairly.
- Credit cards: APR usually labels the interest rate for each transaction type (purchases, cash advances, balance transfers). Many card fees (annual fee, late fee, transfer fee) aren’t folded into that APR number.
Interest rate = raw price of money. APR = interest plus some fees (mainly on loans/mortgages), rolled into a yearly % for comparability.
APR ignores intra-year compounding. APY/EAR includes it. If you carry a balance that compounds daily, your effective annual cost ends up higher than the headline APR.
What Does “Per Annum” Mean—and Why You Feel It Monthly?
“Per annum” literally means per year. The twist is accrual:
- Most cards compute interest daily using a daily periodic rate (APR ÷ 365 or 360).
- Those daily charges accrue and post on your monthly statement.
Quick mental check: If purchase APR is 22.9%, a rough daily rate is 22.9 ÷ 365 ≈ 0.0628%/day. If you carry a balance, compounding those daily charges means your effective annual cost exceeds 22.9%.
Fixed vs Variable APR — Core Differences
| Feature | Fixed APR | Variable APR |
|---|---|---|
| Movement | Doesn’t track an index during the fixed period | Follows a benchmark (e.g., base/prime + margin) |
| Predictability | High | Medium/low (depends on rate path) |
| Typical start | Often slightly higher | Often starts lower |
| Best for | Tight budgets, longer horizons, stability | Short payoff windows, comfort with fluctuation |
| Main risk | Miss savings if rates fall | Costs can jump if rates rise |
| Common uses | Personal loans, fixed-rate mortgages, some cards | Many credit cards, tracker/variable mortgages |
The Four Fine-Print Truths (where UK ≠ US)
1) APR isn’t the “real” cost if you carry a balance
APR is a nominal, annualised label. When you revolve, daily compounding pushes your effective rate above the headline number—hence why balances feel “sticky.”
2) The rate you see isn’t always the rate you get
UK — Representative APR (51% rule)
By FCA rules, the representative APR shown in an advert must be the rate that at least 51% of customers who enter into credit agreements as a result of that promotion will receive. The remaining applicants may be offered a different (often higher) personal APR after underwriting. Always check the representative example: notional amount/limit, term, and total payable.
US — No 51% rule
Ads often show a range (“from X%”). Your offer depends on your credit profile, product tier, and current market.
Bottom line: Treat the ad as marketing. The offer letter is the only rate that matters.
3) “Fixed” on cards isn’t forever—and the rules differ
UK
Providers generally give at least 30 days’ notice before increasing a card interest rate. You usually have a right to reject the change (often 60 days): the account is closed to new spending, and you repay the existing balance at the old rate. You keep historical borrowing protected from the increase but can’t put new purchases on that card.
US (CARD Act)
No APR increase on new purchases in the first 12 months (with specific exceptions). After year one, issuers must give 45 days’ notice before raising the purchase APR. New rates generally apply to future purchases.
14-day nuance: After the 45-day notice is sent, issuers may apply the higher APR to new purchases made 14 or more days after the notice date. Purchases made within the first 14 days after the notice generally remain on the old APR. If you plan to switch or pay down, avoid using the card in that 14-day window.
Loans vs cards
On loans/mortgages, “fixed” generally means fixed for a set period. On cards, “fixed” means not indexed—but issuers can still reprice with proper notice.
4) Not all “APR”s include the same fees (mortgages especially)
- UK/EU mortgages: You’ll see APRC (Annual Percentage Rate of Charge)—designed to fold in interest + most fees across an assumed term. Still, reversion rates, incentives, and early-repayment charges can reshape the real-world cost.
- US mortgages: APR includes interest and many key charges (points, certain closing costs), but assumptions (how long you keep the loan) matter. Two similar APRs can produce very different total paid if you refinance or sell early.
Region Watch-outs (high-value clarifications)
- Business credit cards (US): Many CARD Act consumer protections don’t apply. Terms can be tougher—read them carefully.
- Grace period (both): Pay the prior statement balance in full → most new purchases in the next cycle accrue no interest until the next due date. Carry a balance → you may lose the grace period; purchases can accrue interest immediately. Cash advances and many balance transfers accrue interest immediately (no grace).
- Payment allocation (US): Amounts above the minimum generally go to higher-APR balances first (e.g., cash advances) after the minimum is satisfied—helpful for cutting the priciest slice first.
- Choose Fixed if: you need certainty, expect to hold the debt for a while, or rate rises would strain your budget.
- Choose Variable if: you’ll repay quickly, you can tolerate payment swings, or you expect rates to fall.
- Hybrid tactic: Use a promo (variable/0%) to attack the balance, then fix/switch when fees and market levels line up.
Real-World, Feel-It Examples
A) 12-month plan, steady rates
- If your balance is £/$2,000, cleared in 12 months:
- Fixed APR: predictable payments, easy budgeting.
- Variable APR: may start cheaper, but a late-year increase can erase savings.
B) 24-month plan, rates rise by 1%
Variable’s edge will usually disappear somewhere around a 0.5–1.0% move, depending on your margin, timing, and fees. The longer you revolve, the more compounding magnifies the damage.
What really drives your outcome? 1) Payoff horizon, 2) cash-flow tolerance, 3) fee stack (annual/arrangement, transfer, early-repay, product-switch).
Fees, Promos & Small Print (worth two minutes)
- 0% promos shine only if you clear or meaningfully reduce the balance before revert.
- Balance transfers: a transfer fee + 0% can still beat a high APR—run the maths for your payoff plan.
- Annual/arrangement fees: include them in your horizon comparison, not just the headline APR/APRC.
- Penalty APRs (US): missed payments can trigger much higher rates; expect months of on-time payments before review.
- Representative example (UK): scan it. It’s your shortcut to typical amounts/limits, terms, and total cost.
Behaviour Beats Rate Type (for cost and credit score)
Your real cost is driven more by habits than labels: on-time payments, low utilisation, and avoiding unnecessary new accounts. A low variable can still be expensive if it encourages revolving and fees. Automate payments, set alerts, and shorten the life of any balance you carry.
Pro Tips (that quietly compound in your favour)
- Autopay at least the minimum + a fixed overpayment.
- Mid-cycle payments cut average daily balance (less interest accrues).
- Time big purchases right after your statement date to maximise interest-free days (when a grace period applies).
- Ask for a product switch instead of a new application when you want to fix/change—can reduce fees and avoid a new hard search.
- Set a rate-rise trigger (e.g., “If variable increases by 0.5–1.0%, I switch or accelerate payoff”).
- Mortgages: compare total paid at your actual horizon, not only APR/APRC.
Pros & Cons (at a glance)
- Predictable payments
- Shields you from rate hikes
- Easier budgeting and stress-testing
- Often a slightly higher starting rate
- Exit/product-switch fees may apply
- Miss potential savings if rates fall
- Often the lower starting rate
- Benefit if rates fall
- Suits short payoff windows
- Payment uncertainty
- Reversion/promotional traps
- Rate rises compound costs faster if you revolve
FAQs (exact-match phrasing included)
What is APR?
A yearly cost of borrowing. On loans/mortgages, APR includes interest and certain fees to aid comparison. On cards, APR usually labels the interest rate on transactions; many fees fall outside the APR number.
What does “per annum” mean?
Per year. Cards often compute interest daily and add it monthly—why carrying a balance feels pricier than the headline suggests.
What is per annum?
Same meaning: per year. The nuance is accrual (annual quoting, daily charging).
What does “per annum” mean for interest?
It’s the annualised rate. If you revolve, compounding makes your effective yearly cost exceed the APR.
Is variable APR always cheaper than fixed?
No. It can start cheaper, but rate rises (or a promo ending) can flip the maths. Your payoff speed and fee stack decide a lot.
Can a “fixed” APR on a credit card change?
UK: Expect notice and a right to reject (close to new spend; repay old balance at old rate).
US: Generally no rise in first 12 months; then 45-day notice. Increases usually hit future purchases—remember the 14-day detail on spending right after a notice.
Do all lenders calculate APR the same way?
No. UK/EU mortgages show APRC (Annual Percentage Rate of Charge) to include interest + most fees across an assumed term. US mortgage APR also folds in key charges. Still, compare like-for-like at your horizon.
Do CARD Act rules apply to business cards (US)?
Often not. Many protections don’t extend to business cards; terms can be stricter.
When exactly do I have a grace period?
If you paid the prior statement balance in full, most new purchases in the next cycle carry no interest until the next due date. If you carry a balance, you may lose the grace period for purchases. Cash advances and many balance transfers accrue interest immediately.
- UK “Representative APR” (51% rule): FCA materials define representative APR as a rate that must reflect at least 51% of credit agreements resulting from the advert/promotion.
- US rate changes — 45-day notice & 14-day window: Regulation Z (CARD Act rules) and CFPB commentary illustrate that a higher APR can apply to transactions made 14+ days after the notice is mailed; increases generally apply only to future purchases after the first year.
- General concepts: CFPB explainers on APR vs interest rate; UK/EU APRC usage in mortgage disclosures; mainstream issuer disclosures for daily periodic rate and compounding.
Conclusion
APR is a helpful headline—but not the full story. Read it alongside how interest accrues, whether the rate is fixed or variable, the fee stack, and the local rules (UK vs US). Then choose based on how you actually borrow and how long you’ll keep the debt. That’s how you keep the cost of money predictable—on your terms.