How to Calculate Line of Credit Interest: The Manual Formula That Matches Your Statement
If you want to know how to calculate line of credit interest without waiting for your bank’s monthly statement, the core math is daily simple interest on the average daily balance. The formula is: (average daily balance × APR) ÷ day-count × number of days in the cycle. Most lenders use 365 days, but some business credit lines use 360, which silently raises your cost.
When I first tracked a $75,000 home equity line in 2018, I made the rookie mistake of multiplying the ending balance by the rate. My hand calc was $40 lower than the bank’s charge. The gap was entirely due to a mid-month draw I’d ignored. That error pushed me to build a manual playbook I still use with clients today.
Here is the quick-reference reality check for the amounts people actually search for. At an 8% APR, a $100,000 balance costs about $22.19 per day ($100,000 × 0.08 ÷ 365). On $5,000 at 5% APR, interest is $0.68 per day or roughly $20.55 for a 30-day month. These are daily simple interest figures before payments.
Quick answers: $100k LOC at 8% APR ≈ $22.19/day or ~$665.75/month interest-only. $5k at 5% APR = $0.68/day, ~$20.55/month. $50k monthly payment depends on terms: interest-only at 8% is $333.33; 2% minimum is $1,000 (includes principal).
The thing nobody tells you about line of credit interest is that the “APR” alone is almost useless unless you know your day-count and balance pattern. A variable-rate LOC tied to prime can shift your daily cost overnight, which we’ll cover later. Also note that unlike a credit card, many LOCs do not offer a grace period; interest accrues from the draw date.
Another non-obvious point: interest on a LOC is usually accrued daily but compounded only if unpaid. If you pay the statement balance in full (where required) or at least the interest, you avoid capitalization. But some business lines capitalize accrued interest into principal automatically each month, changing next cycle’s average balance.
What Typical LOC Interest Costs Look Like: $100k, $50k, and $5k Scenarios
Search engines show a lot of calculator widgets for this topic, but they rarely show the worked numbers. Below I break down the three common balances from the People Also Ask boxes so you can sanity-check any tool.
How much interest is on a $100,000 line of credit?
Assume an 8% APR and a 30-day month with a steady balance (no draws or payments). Daily interest = $100,000 × 0.08 ÷ 365 = $21.9178. For 30 days, that’s $657.53. If your lender uses 360 day-count, daily = $22.22, monthly = $666.67 – a $9.14 stealth premium.
If the balance fluctuates, you must use average daily balance, not the peak. For example, if $100k is held for 15 days and $50k for 15 days, average is $75k, interest drops to $493.15 (365-day). That nuance is missing from most bank calculators’ default view. At 10% APR steady, the $100k cost jumps to $821.92/month (365-day).
| APR | Daily (365) | Monthly 30d (365) | Monthly 30d (360) |
|---|---|---|---|
| 6% | $16.44 | $493.15 | $500.00 |
| 8% | $21.92 | $657.53 | $666.67 |
| 10% | $27.40 | $821.92 | $833.33 |
This table is the kind of side-by-side most ranking articles skip. Notice the 360-column always higher. Always ask your lender which row applies.
What is 5% interest on $5,000?
At 5% APR on a $5,000 draw, daily interest = $5,000 × 0.05 ÷ 365 = $0.6849. Over a 30-day billing cycle, you’ll accrue $20.55. If you repay $2,000 on day 10, your average daily balance becomes ($5,000×10 + $3,000×20)/30 = $3,666.67, and interest falls to $15.07. Always weight by days, not by transaction count.
If the same $5,000 sits on a 360-day LOC at 5%, monthly cost is $20.83. The difference is pennies, but scale to $500k and it’s $208 vs $219 – enough to notice.
What is the monthly payment on a $50,000 line of credit?
Payment structure is where lenders diverge. Many personal LOCs require 1%–2% of the balance or interest-only, whichever is higher. At 8% APR, interest-only on $50k is $333.33/month. A 2% minimum payment equals $1,000, of which $666.67 goes to principal. Business lines often demand interest-only plus annual renewals.
I learned the hard way that “minimum payment” does not amortize the line like a term loan. On a $50k draw at 2% min, after a year you’ve paid $12,000 but only reduced principal by ~$8,000 because of compounding draws. Check your contract’s specific sweep rules. Some LOCs require a 30-day “cleanup” period annually where balance must hit zero – missing that triggers a conversion to term loan.
If your rate is variable at prime + 2% (current prime 8.5% → 10.5% APR), interest-only on $50k becomes $437.50. Payment shock is real; model it before drawing.
The Average Daily Balance Playbook: A Real $50k LOC With Mid-Month Draws and a Payment
The biggest content gap among ranking articles is a worked average-daily-balance example with multiple transactions. Here is the exact spreadsheet logic I use for a $50,000 approved line, starting balance $30,000, a $10,000 draw on day 8, a $5,000 payment on day 20, in a 30-day cycle at 7.5% APR (365-day).
Step 1: Map the balance periods. Days 1–7: $30,000 (7 days). Days 8–19: $40,000 (12 days). Days 20–30: $35,000 (11 days). Total days = 30. Sum of (balance × days) = (30k×7)+(40k×12)+(35k×11) = 210,000 + 480,000 + 385,000 = 1,075,000.
Step 2: Divide by 30 to get average daily balance = $35,833.33. Step 3: Daily rate = 0.075 ÷ 365 = 0.00020548. Step 4: Daily interest = $35,833.33 × 0.00020548 = $7.36. Step 5: Month interest = $7.36 × 30 = $220.82.
If you had used ending balance ($35,000) you’d get $215.75 – a $5.07 understatement. Small, but across a year and larger lines, that hides real cost. The Line of Credit Interest Calculator replicates this, but doing it manually once teaches you the weight of timing.
Second Scenario: Three Draws and a Lump Payment
Let’s stress-test with more activity: start $20k, draw $15k day 5, draw $5k day 12, pay $10k day 22, cycle 31 days (January). Segments: days1-4 $20k (4), days5-11 $35k (7), days12-21 $40k (10), days22-31 $30k (10). Balance-days = 80k+245k+400k+300k=1,025,000. Average = $33,064.52. At 6.5% APR, daily rate .00017808, daily int $5.89, month int $182.59.
Doing this by hand took me four minutes. The point is that every draw pushes the average up for the remainder of its days outstanding, not just the day it hits.
Free Spreadsheet Template Structure (Copy Into Google Sheets)
I’ve built a free downloadable template that automates the above. The columns are: Date, Day-Index, Beginning Balance, Draw, Payment, End-of-Day Balance, Days-at-Balance, Balance×Days. A totals row computes sum(Balance×Days) ÷ Cycle-Days = Average Daily Balance. Then a cell for APR and day-count yields interest. This bridges bare formulas and bank widgets.
- Column A: Transaction date (use sequential days if no calendar needed)
- Column B: Days in that balance segment (e.g., 7, 12, 11)
- Column C: Balance during segment
- Column D: =B*C (balance-days)
- Column E: =SUM(D)/SUM(B) for average
- Column F: =E*(APR/365)*SUM(B) for total interest
- Column G: Notes on posting cutoff (e.g., “payment after 5pm ignored”)
Most people don’t realize that a partial payment posted at 4 p.m. may not reduce the day’s balance for that cycle—some lenders cut off at 5 p.m. local time. In my template I flag cutoff timing as a note column because it bit me on a $12k payment that landed a day late in system terms, costing $3.29 extra interest that I successfully disputed.
Day-Count Conventions: The 365 vs 360 Trap That Inflates Your Interest
Consumer LOCs typically use actual/365, but many commercial lines and some credit unions use actual/360. The formula stays identical except the divisor. Using 360 instead of 365 raises effective cost by 1.39% (365/360). On a $100k balance at 8%, that’s about $9 per month as shown earlier.
The Daily Interest Calculator lets you toggle day-count, but in manual work always read your credit agreement’s “how we compute interest” section. I once audited a small-business LOC where the lender used 360 and compounded monthly; the effective APR was 8.22% not 8.00%.
| Convention | Divisor | Effective premium vs 365 | Typical use |
|---|---|---|---|
| Actual/365 | 365 | Baseline | Consumer HELOC, personal LOC |
| Actual/360 | 360 | +1.39% | Commercial LOC, some credit unions |
| Actual/actual | 365/366 | Leap-year adjusted | Municipal or rare bank products |
According to the Consumer Financial Protection Bureau, lenders must disclose how interest is calculated, yet the day-count nuance is often buried in fine print. If you see “bond basis” or “360-day year,” that’s your signal to adjust mental math.
One more wrinkle: some lenders use 365 but exclude the draw day from interest (day-of-funds not counted). That lowers cost marginally. I’ve seen this on two regional banks; it’s worth a call to customer service to confirm.
Variable Rates and How They Break Static Calculations
Most LOCs are variable, priced as prime + margin. When the Federal Reserve moves rates, your APR changes, sometimes mid-cycle. If your cycle spans a rate change, you must split the average daily balance calculation by rate period.
Example: $50k average balance, first 15 days at 7.5%, last 15 at 8.0%. Interest = ($50k×0.075÷365×15) + ($50k×0.08÷365×15) = $154.11 + $164.38 = $318.49. A single blended rate would misstate by a few dollars, but on $1M lines it’s material.
The thing nobody tells you about variable LOCs is that some lenders recalculate the daily rate only at statement close, not daily, creating lag. I’ve seen a client overpay $12 because the bank’s system lagged a prime change by 5 days. Track your own daily rate from the date of public prime announcements (e.g., Fed window).
Also watch for rate floors. A LOC may say “prime minus 0.25%” but floor at 5%. When prime dropped to 3.25% in 2020, my floor kept me at 5% while a naive formula predicted 3%. That’s a 2% overestimation of savings if you didn’t read the contract.
Step-by-Step Manual Calculation Without a Calculator
If you’re asking “How do I calculate interest on my line of credit?” here is the exact manual sequence I teach in workshops. No spreadsheet required, just paper and a calendar.
Step 1: List every balance-changing event
Pull your LOC transactions for the cycle. Note the date and new balance after each draw or payment. Include the starting balance on day 1. Mark the cycle end date.
Step 2: Compute days at each balance
For each segment, count days until the next change. If a draw hits on the 8th, the prior balance ran for 7 days (days 1–7). Be precise about inclusive counting; lenders count each calendar day the balance is outstanding.
Step 3: Multiply and sum balance-days
Multiply each balance by its days. Add them. This sum is the total balance-days. Divide by total cycle days to get average daily balance.
Step 4: Apply the rate and day-count
Convert APR to daily: APR ÷ day-count. Multiply by average daily balance, then by total days. That’s your accrued interest. Cross-check with your statement’s “interest charged” line.
If the numbers differ by more than a few cents, suspect a day-count mismatch or a timing cutoff. I keep a handwritten ledger for the first three cycles of any new LOC to build trust in the bank’s system. This checklist has saved me from three erroneous charges in a decade.
- Verify day-count (365 vs 360) from contract
- Confirm rate period splits if APR changed
- Check payment posting cutoff time
- Recompute average daily balance manually for one cycle
Common Mistakes and Edge Cases I’ve Seen in Practice
Beyond the average-balance oversight, these are the failures that cost real money:
- Assuming interest-only payment zeroes the balance: It doesn’t; you’ll owe the same principal next cycle plus new draws.
- Ignoring the grace period: Some LOCs have none; interest accrues from draw date, not statement date.
- Blending multiple rate periods incorrectly: Always split before averaging if APR changed.
- Using 360-day convention on a 365 loan: Overpays you unknowingly.
- Forgetting annual cleanup clauses: Some LOCs demand zero balance for 30 days yearly.
Most people don’t realize that if you repay and redraw within the same cycle, the average balance may stay high even if your ending balance is low. A $50k draw repaid on day 29 still contributes ~$48k to average. Lenders love this because it maximizes their interest. This is why a LOC can feel like it never gets cheaper even when you “paid it off” before statement.
Another edge case: leap years. In a 366-day February cycle, the daily divisor stays 365 (or 360) but days in cycle = 29. Your formula already multiplies by actual days, so it self-corrects. But some systems prorate weirdly; audit February statements closely. I found a $1.10 discrepancy on a $200k balance in Feb 2020 due to a lender’s legacy 360/366 mix.
Currency fluctuations on cross-border LOCs (e.g., USD line drawn in CAD) add a second layer; interest is in the line currency but your repayment may be converted. Not common for most readers but critical for exporters.
When to Use a Calculator vs Manual Tracking
Manual calculation is best for learning, disputing a charge, or modeling “what-if” draws. For ongoing monitoring, a calculator or our template saves time. But never outsource understanding: a calculator that hides day-count will mask cost.
My honest trade-off: manual ledgers take 10 minutes per cycle; calculators take 10 seconds but can lie if inputs are wrong. I do both for the first year of any new credit line, then trust the calculator with periodic spot-checks. The goal is financial agency, not dependency.
If you manage multiple LOCs, scale the template with a tab per line. The average-daily-balance method is identical; only the transactions differ. In my consultancy, we run a portfolio of 12 LOCs for clients using one master sheet that rolls up interest expense for cash-flow forecasting.
Putting the Playbook to Work Today
You now have the manual formula, a real $50k average-daily-balance walkthrough, day-count warnings, variable-rate splits, and a template structure. Start by printing your last LOC statement and replicating the interest line using the steps above. If it matches, you understand your credit line better than 90% of borrowers.
Then grab the free spreadsheet layout from the earlier section, plug in your transactions, and compare to the bank’s number. The moment you catch a day-count error or a missed payment posting, the exercise pays for itself many times over. Interest is not mysterious; it’s just weighted time and rate.