Open enrollment, with math
Enrollment season asks employees to make three real financial decisions with
zero arithmetic support: which health plan
(/v1/benefits/health-plan), dependent-care FSA or the tax
credit (/v1/benefits/dependent-care), and — while the payroll
portal is open anyway — is the W-4 right
(/v1/tax/w4-checkup)?
1 · HDHP + HSA vs the PPO, in after-tax dollars
The comparison that looks impossible at the benefits fair: a cheap-premium/high-deductible plan with an HSA (and a $1,000 employer seed) against a rich PPO, at $6,000 of expected care.
curl -X POST https://api.numeratica.com/v1/benefits/health-plan \
-H "Authorization: Bearer nmr_sk_test_xxx" \
-H "Content-Type: application/json" \
-d '{
"expected_medical_spend": 6000, "marginal_tax_rate": 0.30,
"hdhp": { "premium": 1200, "deductible": 3000, "oop_max": 6000, "coinsurance": 0.20 },
"ppo": { "premium": 3600, "deductible": 500, "oop_max": 4000, "coinsurance": 0.20 },
"employer_hsa_seed": 1000,
"hsa_contribution": 3000
}'{
"result_id": "healthplan_67765ba76fec7636",
"engine_version": "health-plan-1.0.0",
"result": {
"hdhp": { "premium": 1200, "member_care": 3600, "tax_savings": 900,
"employer_seed": 1000, "net_cost": 2900 },
"ppo": { "premium": 3600, "member_care": 1600, "tax_savings": 0,
"employer_seed": 0, "net_cost": 5200 },
"recommended": "hdhp",
"savings": 2300
},
"disclaimer": "Calculation engine output for informational purposes only; not financial or insurance advice."
}The PPO pays more of the care bill ($1,600 vs $3,600) — and still loses by $2,300, because the premium gap, the HSA tax savings, and the employer seed all sit on the HDHP side. That's the calculation nobody does in the enrollment portal. At higher expected spend the answer can flip — which is exactly why it's an API call, not a rule of thumb.
2 · Dependent-care FSA vs the §21 credit
Same money, two tax treatments, can't double-dip. At $120k MFJ with two kids and $6,000
of care: the FSA's exclusion is worth $1,500, the credit
$1,200 — FSA by $300 (result_id
depcare_1527e0c2c8075b6f). At lower incomes the credit's
higher percentage wins instead; the engine checks, you don't guess.
3 · The W-4 checkup
A single filer at $90k, $6,000 withheld YTD, $500/paycheck, 12 checks left: projected to
be over-withheld by $1,030 — an interest-free loan to the IRS — with the
fix quantified: reduce withholding ~$86/paycheck, and the §6654 safe
harbor is still cleared ($9,873 target) (result_id
w4_a861711fdccf8dba). Runs the full tax engine underneath, so
credits, QBI, and itemizing all flow through if given.
4 · Render it
The two-plan card for the featured call.
<div style="font-family:-apple-system,Segoe UI,Roboto,Helvetica,Arial,sans-serif;color:#1a2230;max-width:600px">
<div style="font-size:14px;color:#5b6675;margin-bottom:8px">Expected care $6,000 · 30% marginal rate · after-tax annual cost</div>
<div style="display:grid;grid-template-columns:1fr 1fr;gap:10px">
<div style="border:2px solid #1d4ed8;border-radius:10px;padding:12px">
<div style="font-size:12px;color:#1d4ed8;font-weight:600">HDHP + HSA · recommended</div>
<div style="font-size:22px;font-weight:700">$2,900<span style="font-size:12px;font-weight:400;color:#5b6675">/yr net</span></div>
<div style="font-size:12px;color:#5b6675;margin-top:4px">$1,200 premium + $3,600 care − $900 tax − $1,000 seed</div>
</div>
<div style="border:1px solid #e6e9ef;border-radius:10px;padding:12px">
<div style="font-size:12px;color:#5b6675">PPO</div>
<div style="font-size:22px;font-weight:700">$5,200<span style="font-size:12px;font-weight:400;color:#5b6675">/yr net</span></div>
<div style="font-size:12px;color:#5b6675;margin-top:4px">$3,600 premium + $1,600 care · no tax break</div>
</div>
</div>
<div style="margin-top:10px;border:1px solid #16a34a;background:#f0fdf4;border-radius:10px;padding:12px;display:flex;justify-content:space-between;align-items:baseline">
<div><strong>HDHP wins here</strong><span style="color:#5b6675;font-size:13px"> · even while paying more of the care bill</span></div>
<div style="font-size:22px;font-weight:700;color:#16a34a">saves $2,300</div>
</div>
<div style="color:#5b6675;font-size:12px;margin-top:6px">result_id healthplan_67765ba76fec7636 · not insurance advice</div>
</div>
Where the freed-up dollars go next: the savings waterfall (the HSA is step 2). Lumpy income instead of a W-2? The safe-harbor guide is the W-4 checkup's sibling.