Why interest results differ in Texas

When you run the same Texas interest calculation in two places and get two different answers, it’s rarely “math magic.” It’s almost always a hidden input, assumption, or jurisdiction rule.

This post gives you a quick diagnostic for why results differ in Texas, and how to pin down the mismatch using DocketMath’s /tools/interest calculator.

The top 5 reasons results differ

When comparing interest results (e.g., your spreadsheet vs. DocketMath, or two different tools), start by checking these five variables.

• Different trigger dates or event definitions were used.
• Inputs were entered with different day-count or compounding assumptions.
• Payments, credits, or tolling periods were handled differently.
• Jurisdiction or court settings did not match the matter.
• Rounding or cutoff-time rules were applied inconsistently.

1. Simple vs. compound interest

Texas law often uses simple interest for common judgment and pre‑judgment scenarios, but many generic calculators default to compound interest (monthly or annually).

• Simple interest: interest = principal × rate × time
• Compound interest: interest = principal × (1 + rate/periods)^(periods×time) − principal

If one tool compounds and the other doesn’t, you’ll see:

• Small differences for short periods
• Very large gaps over multi‑year periods

What to check in DocketMath:

• Confirm the interest type (simple vs. compound) setting.
• If comparing to a generic “finance” calculator, assume it may be compounding unless explicitly stated.

2. Day-count convention (how days are measured)

Even with the same rate and dates, different day-count conventions change the output:

• Actual/365 – uses the actual number of days, divides by 365
• Actual/360 – uses actual days, divides by 360 (effectively a higher yield)
• 30/360 – assumes each month is 30 days, year is 360 days

Texas-specific rules or contracts may call for one method, while:

• Spreadsheets (like Excel’s built-ins) may use 30/360.
• Finance-oriented tools may default to Actual/360.

What to check in DocketMath:

• Look for the day-count or basis field and match it to:

  • The other tool’s documentation, or
  • The contract / statute you’re modeling.

3. Start date vs. end date (inclusion rules)

A very common hidden mismatch: whether the calculation includes the start date, the end date, both, or neither.

For example, interest from Jan 1 to Jan 31 can be treated as:

• 30 days (exclude end date),
• 31 days (include end date), or
• 0 days (if both ends are excluded due to a data-entry error).

Different Texas contexts may specify “from and after” a date or “through” a date, and tools implement those phrases differently.

What to check in DocketMath:

• Confirm:

  • The start date is the date interest begins accruing.
  • The end date setting matches the legal or contractual rule (e.g., up to but not including the payment date, or through the payment date).
• Manually count the days for a short test period to see which convention the other tool is following.

4. Wrong or stale interest rate

Texas has multiple interest regimes depending on context (e.g., post‑judgment, contract, statutory caps). Two tools may:

• Use different base rates (e.g., an older statutory rate vs. a current one).
• Apply caps or floors differently.
• Use fixed vs. variable (periodically resetting) rates.

You can get the same “type” of interest (simple, same dates, same day-count) but still disagree if:

• One tool hard-codes a historical rate.
• Your spreadsheet uses a static rate but the rule requires periodic updates.

What to check in DocketMath:

• Verify the annual rate you’ve entered (or that DocketMath is using for Texas).
• Confirm whether the scenario calls for:

  • A fixed rate across the whole period, or
  • A changing rate (e.g., tied to a published index).

5. Principal changes, payments, and fee layering

If the principal isn’t constant, calculators can diverge quickly:

• Interim payments (partial or full).
• Additional principal (e.g., added costs, fees, or advances).
• Whether fees themselves accrue interest.

Some tools assume:

• Interest runs on the original principal only.
• Payments always apply to interest first, then principal.
• No interest on fees or costs unless explicitly included.

What to check in DocketMath:

• Use the events or transactions inputs:

If the other tool doesn’t show a transaction timeline, that’s a strong sign its internal assumptions differ from what you’re modeling.

How to isolate the variable

When two results don’t match, simplify until they do, then add complexity back in one step at a time.

• Freeze the jurisdiction and tool settings so both runs use the same rule set.
• Compare one input at a time (dates, rates, amounts) and re-run after each change.
• Review the breakdown to see which segment or assumption drives the difference.

Step 1: Build a “toy case”

Create a very simple scenario that both tools can handle:

• Principal: $1,000
• Rate: 10% simple
• No payments or additional charges
• Period: exactly 1 year (e.g., Jan 1 to Dec 31 under each tool’s default)

Run this in:

• Your spreadsheet or comparison tool
• DocketMath’s /tools/interest calculator

If the results differ here, the cause is likely:

• Simple vs. compound, and/or
• Day-count convention, and/or
• Start/end date inclusion

Step 2: Change one input at a time

Once you have agreement on the toy case, add complexity stepwise:

1. Change the rate (e.g., from 10% to your actual Texas rate).
2. Change the dates to your real start/end dates.
3. Switch the day-count method (if your scenario requires it).
4. Add one payment and compare.
5. Add all payments / fees and re-check.

At each step:

• If the numbers diverge, the last change is your suspect.
• Document the setting that caused the break.

Step 3: Document your assumptions

For each final calculation, keep a short assumptions list:

• Interest type: simple / compound (and compounding frequency, if any)
• Day-count convention
• Start and end date inclusion rule
• Rate source (e.g., “Texas X statute as of [date]”)
• Payment application rule (interest-first vs. principal-first)

This won’t answer legal questions, but it will make your math reproducible and easier to defend.

Next steps

When you see mismatched Texas interest results:

1. Identify which of the five variables is most likely off:
2. Use a toy case in DocketMath’s /tools/interest tool to align on basics.
3. Layer in complexity one input at a time until you find the divergence.
4. Treat legal questions as legal questions: which rate applies, whether interest can be compounded, and how to treat specific dates or charges are all issues for legal judgment, not the calculator.
5. Save your setup (screenshots, PDFs, or a written assumptions sheet) so you can repeat or explain the calculation later.

Related reading

• A practical workflow for jurisdiction-aware legal calculations (and how to document them)

• Why we started the DocketMath blog

• Why we started the DocketMath blog