Financial planning & wealthtech infrastructure
A tax calculation API for planning products
A tax calculation API computes tax outcomes from structured inputs — an option exercise, a Roth conversion, a capital gain — so products can answer tax questions without maintaining the statutes themselves. Worthune's tax models are planning-grade and federal (plus state income tax): AMT on an ISO exercise to the share, QSBS §1202 across its regimes, 83(b) elections, wash sales, capital gains, quarterly estimates, and the retirement-income cliff stack where Social Security taxation, Medicare surcharges, and ACA subsidies interact. Every constant is sourced to the Revenue Procedure, statute, or agency table that set it.
What to look for — whoever you choose
An honest scope statement
Planning math and filing software are different products. A trustworthy tax API states plainly which it is, and publishes what each model deliberately excludes — the exclusions section is where you learn whether a vendor respects the boundary of its own claim.
Statute-sourced constants
Brackets, exemptions, phaseouts, and thresholds should cite the primary source and carry the date a human last checked it — and update on the source's calendar, not the vendor's.
Interaction handling
The expensive tax errors are interactions: a conversion that crosses an IRMAA threshold two years out, two rule sets defining the same word (MAGI) differently. Ask how the API models the cliffs, not just the brackets.
How Worthune answers it
The tax models, spec'd for skeptics
Each publishes formulas, assumptions, and exclusions — the AMT model states in plain words where it understates. Planning-grade, federal, never filing software, and it says so.
→ see the tax familyConstants from the source
IRS, SSA, CMS, and HHS figures in a public registry with primary sources, effective periods, and check dates — cited in every response that uses them.
→ browse the facts registryThe cliff math, worked
The retirement-income cliffs model computes the true marginal rate across §86 Social Security taxation, Medicare tiers, and ACA subsidies — the interaction spreadsheets can't hold.
→ read the specWhere this fits: everything above is one platform — calculators & models, planning software, and unified household management, on the same verified engines and one API. How the numbers are made is public: the methodology.
Common questions
Is this filing-grade tax software?
No — planning-grade, and deliberately so. The models answer questions like "exercise now or in January" and "what does this conversion actually cost," with each spec publishing its exclusions. They do not prepare returns and never claim to.
Which taxes does the API cover?
Federal planning math — AMT on ISO exercises, QSBS, 83(b), wash sales, capital gains, quarterly estimates, retirement-income cliffs — plus state income tax. Each model's contract states its exact coverage and exclusions.
How do the numbers stay current each tax year?
Constants update from primary sources on their own calendars — brackets and AMT in the fall, Medicare tiers in November, poverty guidelines in January — each change shipping as a public spec version bump. You can pin a version in your integration.
Can a compliance reviewer check the math?
Yes, without asking permission: specs are published, constants cite their sources, and every response carries a SHA-256 evidence record that can be recomputed later to prove nothing moved.
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