“How to Calculate Credit Card Interest” (3 Steps to Find Your Rate)

credit card advice

Eric Bank
By: Eric Bank
Posted: February 28, 2017
Opinions expressed here are ours alone, and are not provided, endorsed, or approved by any issuer. Site may be compensated through the issuer affiliate programs.

One of the most important factors in choosing a credit card is its interest rate. All credit cards must disclose their annual percentage rate, or APR, which expresses their interest rates in a single annual number. Almost all credit cards charge interest only on balances that you don’t fully pay off in the most current billing cycle. But how does APR translate into the amount of interest you’ll actually pay?

Read on to see the steps, or skip ahead to learn how your APR is calculated or to compare the best balance transfer offers to avoid paying interest.

Step 1: Divide APR by 360 (or 365) to Find Daily Periodic Rate

Before we can explain the first step, we should clarify a few important terms:

Grace Period: The normal billing cycle for a credit card can range from 28 to 31 days. The due date for your monthly payment is no less than 21 days after the end of the billing cycle. Each billing cycle provides a grace period, which means you don’t incur interest on purchases made within the billing cycle if you pay them in full by the due date. Your outstanding balance is the accumulated amount you haven’t paid within a grace period.

Simple Versus Compound Interest: Simple interest is the amount you pay on your outstanding balance without including the effect of compounding. Virtually all cards figure your interest with compounding, which means they add the interest you already owe to the amount subject to interest – you are paying interest on interest.

In the good old days, credit cards used monthly compounding, but the current fashion is daily or continuous compounding, which will cost you more. As an example of daily compounding, if your outstanding balance is $1,000 and the day’s interest is 71 cents, then tomorrow’s outstanding balance will be $1,000.71 (assuming no other purchases or payments).

Nominal Versus Effective APR: When you see an ad for a credit card, the interest rate is expressed as the nominal APR, which is based on simple interest and excludes fees. The more appropriate number is the effective APR, which includes the effects of compounding and any fees that are not paid separately. Some cards charge the annual fee (if any) as a lump sum, but others spread the annual fee over the entire year, making it part of the effective APR.

If possible, obtain the effective APR of any credit card you are considering. Fees for late payments or for exceeding your credit limit are not included in any APR, since they are charged separately.

To illustrate the three-step process for calculating your interest charges, imagine that you have an outstanding balance of $3,500 on a credit card with an interest rate of 25 percent.

In this example, the credit card uses a 360-day year (some cards use 365, terms will vary), so the daily percentage rate, or DPR, is equal to 25% / 360, or .06944%. This is the interest rate you pay each day on the balance subject to interest. Assuming daily compounding and no other activity, your daily balance would grow each day by the interest charged on the previous day.

Step 2: Calculate Your Average Daily Balance

The average daily balance is computed by adding together each day’s outstanding balance and dividing by the number of days in the billing period. For simplicity’s sake, we set the average daily balance to $3,500.

Step 3: (Avg. Daily Balance x DPR) x Days in the Month

Finally, we calculate the interest charged for the billing cycle, which in this example, is $3,500 x .06944% x 30 days, or $72.91. This is the amount of interest you would be charged on a card with a $3,500 balance and a 25% interest rate.

How Banks Determine Your APR

The APR on your credit card is based on the bank’s opinion of your creditworthiness, which is in large part derived from your credit score. Most banks use FICO credit scores, which range from 300 (the worst) to 850 (the best).

Each of the three major credit bureaus, Equifax, Experian, and TransUnion figure their FICO scores a little differently, but the scores tend to cluster closely. Interpretations differ, but generally, 700 is considered the dividing line between good and fair scores. The lower the score, the higher APR you’ll pay. If your score is too low, you could have difficulty qualifying for any credit at all.

Sometimes, banks will take into account other factors when determining your APR, such as black marks on your credit history (bankruptcies, court decisions, garnished wages, etc.). You will save money on interest if you can raise your FICO score – check out the myFICO website for tips on how to do that.

Delay Interest Payments with a Balance Transfer Offer

Many credit cards offer special balance transfer deals that can save you money. In a balance transfer, you move your outstanding balance from one credit card to another. To entice new customers, the balance transfer offers usually include a set number of months during which you don’t owe any interest on the transferred amount.

Some of the best balance transfer offers grant interest-free periods of 15 or even 21 months. In addition, some cards will also grant a multi-month grace period on new purchases made after you transfer a balance. If you combine that with a zero-percent annual fee and a reasonably low APR, you have the makings of a great credit card. Here are some of our favorites:

Discover it® - 18 Month Balance Transfer Offer Review

at Discover Card'ssecure website

0% BALANCE TRANSFER RATING

★★★★★
5.0

OVERALL RATING

4.9/5.0
  • INTRO OFFER: Discover will match ALL the cash back earned at the end of your first year, automatically.
  • Earn 5% cash back at different places each quarter like gas stations, grocery stores, restaurants, Amazon.com, or wholesale clubs up to the quarterly maximum each time you activate.
  • Redeem your cash back for any amount, any time. Cash rewards never expire.
  • 100% U.S. based customer service.
  • Get your free Credit Scorecard with your FICO® Credit Score, number of recent inquiries and more.
  • New! Receive FREE Social Security number alerts-Discover will monitor thousands of risky websites when you sign up.
Intro (Purchases)
Intro (Transfers)
Regular APR
Annual Fee
Credit Needed
0% for 6 months
0% for 18 months
11.99% - 23.99% Variable
$0
Good/Excellent

0% BALANCE TRANSFER RATING

★★★★★
4.9

OVERALL RATING

5.0/5.0
  • Unlimited 1.5% cash back on every purchase — it's automatic
  • Earn a $150 Bonus after you spend $500 on purchases in your first 3 months from account opening
  • 0% Intro APR for 15 months from account opening on purchases and balance transfers, then a variable APR of 15.99-24.74%. Balance transfer fee is 5% of the amount transferred, $5 minimum
  • No minimum to redeem for cash back
  • Cash Back rewards do not expire as long as your account is open
  • No annual fee
Intro (Purchases)
Intro (Transfers)
Regular APR
Annual Fee
Credit Needed
0% for 15 months
0% for 15 months
15.99% - 24.74% Variable
$0
Excellent/Good
Discover it® - Cashback Match™ Review

at Discover Card'ssecure website

0% BALANCE TRANSFER RATING

★★★★★
4.9

OVERALL RATING

5.0/5.0
  • Earn 5% cash back at different places each quarter like gas stations, grocery stores, restaurants, Amazon.com, or wholesale clubs up to the quarterly maximum each time you activate.
  • INTRO OFFER: Discover will match ALL the cash back earned at the end of your first year, automatically.
  • Redeem your cash back for any amount, any time. Cash rewards never expire.
  • Use your rewards instantly at Amazon.com checkout.
  • No annual fee.
  • Click "APPLY NOW" to see rates, rewards, FICO® Credit Score terms, Cashback Match™ details & other information.
Intro (Purchases)
Intro (Transfers)
Regular APR
Annual Fee
Credit Needed
0% for 14 months
0% for 14 months
11.99% - 23.99% Variable
$0
Good/Excellent

0% BALANCE TRANSFER RATING

★★★★★
4.9

OVERALL RATING

4.7/5.0
  • $0 Introductory balance transfer fee for transfers made during the first 60 days of account opening
  • 0% Introductory APR for 15 months on purchases and balance transfers
  • Monthly FICO® Score and Credit Dashboard for free
  • No Penalty APR – Paying late won't raise your interest rate (APR). All other account pricing and terms apply
  • $0 Annual Fee
Intro (Purchases)
Intro (Transfers)
Regular APR
Annual Fee
Credit Needed
0% for 15 months
0% for 15 months
15.99%-24.74% Variable
$0
Excellent Credit
Wells Fargo Platinum Visa® Card Review

at Wells Fargo'ssecure website

0% BALANCE TRANSFER RATING

★★★★★
4.9

OVERALL RATING

4.6/5.0
  • 0% Intro APR for 18 months on purchases and balance transfers, then a 16.15%-25.99% variable APR; balance transfer fees apply
  • Get up to $600 protection on your cell phone (subject to $25 deductible) against covered damage or theft when you pay your monthly cellular telephone bill with your Wells Fargo Platinum Visa® Card
  • Free access to your FICO® Credit Score with Wells Fargo Online®
  • Zero Liability protection for promptly reported unauthorized transactions
  • Convenient tools to help create a budget and manage your spending with My Money Map
  • $0 Annual Fee
Intro (Purchases)
Intro (Transfers)
Regular APR
Annual Fee
Credit Needed
0% for 18 months
0% for 18 months
16.15%-25.99% (Variable)
$0
Excellent/Good

+See more balance transfer cards

Pay Your Balance in Full to Avoid Interest Altogether

Now that you know how interest is calculated, you can follow this strategy to lower your interest expenses:

  1. Improve your credit score
  2. Shop around for a credit card that combines the lowest effective APY with the best balance transfer program
  3. Transfer debt from other credit cards to your new one
  4. Pay down your credit card debt during the 0% interest intro period of your balance transfer
  5. Avoid accumulating outstanding balances in the future

For almost all credit cards, you’d never shell out any interest if you paid off your balance in full each month. However, some credit cards do not provide grace periods, so always check. One other point – cash advances incur interest immediately and never have a grace period.