Worked example: interest in Massachusetts
7 min read
Published February 1, 2026 • Updated February 2, 2026 • By DocketMath Team
Worked example: interest in Massachusetts
Calculating interest on a judgment in Massachusetts looks simple on paper—“12% per year” is the number most litigators know—but the details matter:
- What date do you start from?
- Do you use calendar days or months?
- How do partial payments affect the math?
- How do you sanity‑check the output?
This walkthrough shows how a realistic Massachusetts example looks inside DocketMath’s interest calculator, and how small changes in the inputs change the result.
Note: This post is about how to use a calculator and interpret its numbers, not about what you should claim or what a court must award. Always confirm applicable rates, dates, and rules for your case and jurisdiction.
Example inputs
Suppose you represent a plaintiff in a contract case in Massachusetts state court. You won a money judgment, and now you want to understand post‑judgment interest for a potential settlement discussion.
Here’s the basic fact pattern we’ll use:
- Jurisdiction: Massachusetts (US‑MA)
- Principal amount (judgment): $150,000
- Interest type: Post‑judgment interest
- Statutory rate: 12% simple interest per year (Massachusetts’ default rate for many civil judgments)
- Judgment date: March 15, 2022
- As‑of date (calculation date): January 31, 2025
- Compounding: None (simple interest)
- Partial payments: One mid‑stream payment
We’ll also assume:
- The defendant made one partial payment of $40,000 on July 1, 2023.
- No other credits, costs, or attorney’s fees are included in the interest base.
- Interest is calculated using actual days elapsed / 365 (a common convention for simple statutory interest).
These are the key inputs you’d enter into DocketMath:
| Input field | Value | Why it matters |
|---|---|---|
| Jurisdiction | Massachusetts (US‑MA) | Drives the default rate and rules assumptions |
| Principal amount | 150,000 | Base amount that earns interest |
| Interest type | Post‑judgment | Determines which dates and rate to use |
| Annual interest rate | 12% (simple) | Massachusetts default for many civil judgments |
| Judgment date | 2022‑03‑15 | Start date for post‑judgment interest |
| As‑of date | 2025‑01‑31 | End date for the calculation |
| Compounding | None | MA judgment interest is typically simple, not compounded |
| Partial payment date | 2023‑07‑01 | Date when the principal is reduced |
| Partial payment amount | 40,000 | Amount that stops accruing interest after payment |
| Day‑count convention | Actual/365 | How the tool converts days into a fraction of a year |
Pitfall: If you enter the filing date or breach date instead of the judgment date for post‑judgment interest, your number can be off by years of interest. Be clear whether you’re modeling pre‑judgment or post‑judgment interest.
Example run
We’ll walk through the calculation in two segments: before the partial payment and after the partial payment. This mirrors how DocketMath internally breaks the timeline when you add payments or principal changes.
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: Days and interest before the partial payment
Segment 1: From judgment date (2022‑03‑15) to payment date (2023‑07‑01)
Principal during this segment: $150,000
Count the days
2022‑03‑15 to 2023‑03‑15: 365 days (non‑leap year)
2023‑03‑15 to 2023‑07‑01:
- March 15–31: 17 days
- April: 30 days
- May: 31 days
- June: 30 days
- July 1: 1 day
- Subtotal: 17 + 30 + 31 + 30 + 1 = 109 days
Total days in Segment 1:
365 + 109 = 474 days
Convert days to years
Using Actual/365:
[ \text{Years}_1 = \frac{474}{365} \approx 1.2993 \text{ years} ]
Compute interest for Segment 1
Formula (simple interest):
[ \text{Interest} = \text{Principal} \times \text{Rate} \times \text{Time in years} ]
Plug in:
[ \text{Interest}_1 = 150{,}000 \times 0.12 \times 1.2993 ]
[ 150{,}000 \times 0.12 = 18{,}000 ]
[ 18{,}000 \times 1.2993 \approx 23{,}387.40 ]
- Segment 1 interest: ≈ $23,387.40
In DocketMath, this appears as this as a line item:
- 2022‑03‑15 → 2023‑07‑01
- Principal: $150,000
- Days: 474
- Interest: ≈ $23,387.40
Step 2: Apply the partial payment
On 2023‑07‑01, the defendant pays $40,000.
Under a simple model where payments first reduce principal (not accrued interest), you’d treat the principal as:
- Before payment: $150,000
- After payment: $150,000 − $40,000 = $110,000
Depending on your legal theory or judgment language, you might apply payments to interest first. DocketMath lets you choose the allocation method, but for this example we’ll use the straightforward “apply to principal” approach so the timeline is easy to see.
Step 3: Days and interest after the partial payment
Segment 2: From 2023‑07‑01 to 2025‑01‑31
Principal during this segment: $110,000
Count the days
2023‑07‑01 to 2024‑07‑01: 366 days (includes 2024‑02‑29; 2024 is a leap year)
2024‑07‑01 to 2025‑01‑31:
- July: 31 days
- August: 31 days
- September: 30 days
- October: 31 days
- November: 30 days
- December: 31 days
- January 1–31: 31 days
- Subtotal: 31 + 31 + 30 + 31 + 30 + 31 + 31 = 215 days
Total days in Segment 2:
366 + 215 = 581 days
Convert days to years
Again using Actual/365:
[ \text{Years}_2 = \frac{581}{365} \approx 1.592 \text{ years} ]
Compute interest for Segment 2
[ \text{Interest}_2 = 110{,}000 \times 0.12 \times 1.592 ]
[ 110{,}000 \times 0.12 = 13{,}200 ]
[ 13{,}200 \times 1.592 \approx 21{,}014.40 ]
- Segment 2 interest: ≈ $21,014.40
In DocketMath, this appears as:
- 2023‑07‑01 → 2025‑01‑31
- Principal: $110,000
- Days: 581
- Interest: ≈ $21,014.40
Step 4: Total interest and payoff amount
sum the segments:
Total interest
[ \text{Interest}_\text{total} = 23{,}387.40 + 21{,}014.40 \approx 44{,}401.80 ]Total collected so far
- Partial payment: $40,000
Outstanding principal (under our “payment to principal” assumption)
- Original principal: $150,000
- Less payment: $40,000
- Remaining principal: $110,000
Total payoff as of 2025‑01‑31
[ 110{,}000 + 44{,}401.80 \approx 154{,}401.80 ]
So, as of January 31, 2025, DocketMath would report something like:
- Remaining principal: $110,000
- Accrued interest: ≈ $44,401.80
- Total payoff: ≈ $154,401.80
DocketMath can generate this breakdown in the DocketMath interest tool by:
- Selecting Massachusetts (US‑MA) as the jurisdiction.
- Entering $150,000 as the principal, 12% as the annual rate.
- Setting judgment date = 2022‑03‑15, as‑of date = 2025‑01‑31.
- Adding a partial payment of $40,000 on 2023‑07‑01.
- Confirming no compounding and an Actual/365 basis.
Sensitivity check
Once you have a baseline calculation, the most useful thing you can do is test a few quick variations so you know which assumptions move the number.
- Change the rate (for example, 12% → 8% or 15%).
- Shift the as‑of date forward 30–90 days.
- Switch the payment allocation (interest‑first vs. principal‑first) and compare totals.
In DocketMath, duplicate the run and adjust one input at a time. The delta tells you which assumption is doing the most work.