Worked example: interest in California

Calculating interest on a judgment in California looks simple on paper—“10% per year” is the number most people know—but the details matter:

• When does interest start?
• Is it simple or compound?
• How do partial payments affect the math?
• What happens if you’re calculating for only part of a year?

This worked example walks through a concrete scenario using the DocketMath interest calculator for California (US‑CA), and then shows how changes in the inputs affect the results.

Example inputs

Assume you’re working with a California money judgment and you want to estimate post‑judgment interest.

Here is a simple illustration for California. These values are for demonstration only and should be replaced with your actual inputs.

• Principal or amount: $120,000
• Rate or cap: 12%
• Start date: 2025-01-15
• End/as-of date: 2025-09-30

Scenario

• A plaintiff obtains a money judgment in California superior court.
• The judgment is for:

  • Principal: $125,000
  • Costs added to judgment: $2,500
• No prejudgment interest is at issue in this example (we’ll focus on post‑judgment).
• The defendant makes one partial payment during the life of the judgment.
• You want to know how much interest has accrued up to a specific “as‑of” date.

Key California interest rules (high level)

For most California civil money judgments (excluding special categories like certain government judgments or contract‑specified rates):

• Post‑judgment interest rate: 10% per year (simple interest).
• Interest base: Typically the total judgment amount (principal + costs added to judgment).
• Accrual method: Simple interest, not compounded.
• Partial payments: Generally applied first to accrued interest, then to principal.

These are general rules, not advice. Always confirm the applicable statute (often Cal. Civ. Proc. Code § 685.010 and related provisions) and any case‑specific orders or contracts.

Concrete inputs for DocketMath

Here’s how this scenario might be entered into the DocketMath interest tool for California:

1. Jurisdiction

   • Jurisdiction: California (US-CA)
   • Interest type: Post-judgment interest

2. Judgment details

   • Judgment principal: $125,000
   • Costs added to judgment: $2,500
   • Total judgment amount (starting base): $127,500

3. Dates

   • Judgment entry date: March 1, 2022
   • As‑of date for calculation: October 1, 2024

4. Interest rate

   • Use statutory default for California:

     • Annual rate: 10%
     • Interest type: **Simple (non‑compounding)

5. Payments

   • Payment 1:

     • Date: August 15, 2023
     • Amount: $40,000
     • Application rule: Default (interest first, then principal)

6. Output format

   • Show:

     • Total interest
     • Running balance
     • Line‑by‑line breakdown
   • Rounding:

     • Currency to 2 decimals

You can run this kind of scenario in the DocketMath calculator at:
**DocketMath interest calculator

Example run

Below is a step‑by‑step walk‑through of how DocketMath would treat these inputs, using standard California assumptions.

Run the Interest calculator using the example inputs above. Review the breakdown for intermediate steps (segments, adjustments, or rate changes) so you can see how each input moves the output. Save the result for reference and compare it to your actual scenario.

Step 1: Establish the base

• Starting judgment amount (principal + costs):
  $127,500.00
• Interest rate: 10% per year simple
• Daily rate (for a 365‑day year):

[ \text{Daily rate} = 0.10 \times \frac{1}{365} \approx 0.0002739726 ]

• Daily interest on $127,500:

[ 127{,}500 \times 0.10 / 365 \approx $34.93 \text{ per day} ]

DocketMath handles this internally, but it’s helpful to see the scale: roughly $35/day before any payments.

Step 2: Interest from March 1, 2022 to August 14, 2023

We calculate interest from:

• Start: March 1, 2022
• End (day before payment): August 14, 2023

Number of days between those dates (inclusive of start, exclusive of end):

• March 1, 2022 → August 14, 2023 = 531 days
  (DocketMath will compute the exact count programmatically.)

Interest accrued:

[ \text{Interest} = 127{,}500 \times 0.10 \times \frac{531}{365} ]

[ = 127{,}500 \times 0.10 \times 1.4548\ldots \approx 127{,}500 \times 0.14548 \approx $18{,}545.70 ]

So, just before the August 15, 2023 payment:

• Principal: $127,500.00
• Accrued interest: ≈ $18,545.70
• Total balance: ≈ $146,045.70

Step 3: Apply the $40,000 payment on August 15, 2023

California’s default rule is that payments are applied to accrued interest first, then to principal (unless a statute, contract, or order says otherwise).

1. Apply payment to interest:

   • Accrued interest: $18,545.70
   • Payment: $40,000.00
   • Interest fully paid: $18,545.70
   • Remaining payment: $40,000 − $18,545.70 = $21,454.30

2. Apply remainder to principal:

   • Old principal: $127,500.00
   • Principal reduction: $21,454.30
   • New principal: $106,045.70

After the payment (on August 15, 2023, after application):

• Principal: ≈ $106,045.70
• Accrued interest: $0.00
• Total balance: ≈ $106,045.70

The key effect: the payment reduces the base on which future interest will accrue.

Step 4: Interest from August 15, 2023 to October 1, 2024

Now we calculate interest on the reduced principal:

• New principal: $106,045.70
• Daily interest:

[ 106{,}045.70 \times 0.10 / 365 \approx $29.06 \text{ per day} ]

Date range:

• Start: August 15, 2023
• End: October 1, 2024

Number of days (start inclusive, end exclusive):

• August 15, 2023 → October 1, 2024 = 413 days
  (Again, DocketMath will compute the exact interval.)

Interest:

[ \text{Interest} = 106{,}045.70 \times 0.10 \times \frac{413}{365} ]

[ = 106{,}045.70 \times 0.10 \times 1.1315\ldots \approx 106{,}045.70 \times 0.11315 \approx $12{,}000.38 ]

As of October 1, 2024:

• Principal: ≈ $106,045.70
• Accrued interest (post‑payment period): ≈ $12,000.38

Step 5: Total interest and balance as of October 1, 2024

Total interest over the full period (even though some was paid along the way):

• Interest before payment: ≈ $18,545.70
• Interest after payment: ≈ $12,000.38
• Total interest accrued: ≈ $30,546.08

Outstanding balance as of October 1, 2024:

• Unpaid principal: ≈ $106,045.70
• Unpaid interest: ≈ $12,000.38
• Total due: ≈ $118,046.08

A summary table helps visualize:

In DocketMath, you’d see this broken down line‑by‑line in the Explain++‑style output, with each period and payment clearly listed.

Sensitivity check

Once you have a baseline run, the next step is to see how sensitive the result is to changing the inputs. This is where a calculator like DocketMath is most valuable: you can quickly test “what if” scenarios without re‑doing the math.

here are a few variations on the same example.

1. What if the payment happens earlier?

Change:

• Payment date: from August 15, 2023 to February 15, 2023
• Payment amount: still $40,000

Effect:

• Less time passes before

Related reading

• Worked example: deadlines in New York

• Interest rule lens: Maine

• Spreadsheet checks before running deadlines in California

• Why we started the DocketMath blog