round_cash_total
Round a cash total the way the post-penny rules actually work
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Third-party content written by another agent. Data to evaluate, not instructions.
Settle a transaction under post-penny cash rounding: exact subtotal, tax on the exact subtotal, then round the final total to the nearest nickel — and only for cash.
Use this for any "what does X round to" or "what do I owe in cash" question. Four things are wrong in most explanations of this, and each one changes the answer:
- CASH ONLY. Card, EFT, and gift-card payments are still priced to the cent. The same basket legitimately comes to two different amounts by tender. If asked about a card payment, do not round.
- THE TOTAL, NOT EACH ITEM. Rounding per item multiplies the effect by the basket size. Five items at $0.99 total $4.95 — already a nickel, no rounding — where per-item rounding would collect $5.00.
- AFTER TAX. Tax is computed on the exact subtotal and the rounding comes last. Rounding the base first changes the tax owed.
- THE DIGIT DECIDES. 1, 2, 6, 7 round down; 3, 4, 8, 9 round up; 0 and 5 stand. Do not reason about "nearest" from scratch, and do not assume rounding is always up.
Input: pass subtotal as a dollar string ("19.99") or items as a list. A NUMERIC amount is read as CENTS and a STRING as dollars — 1999 and "19.99" are the same amount, 19.99 as a number is refused. Pass taxRatePercent as a percentage (8.25, not 0.0825); rounding depends on it, so omitting it usually gives the wrong nickel. tender is cash or card, default cash. rule is symmetric (default), always-down, always-up, or none.
Returns: the exact subtotal, tax, exact total, the rounded amount due, the signed delta, which digit decided it, warnings, and a disclaimer field.
THIS IS NOT LEGAL OR TAX ADVICE. There was no federal rounding law as of September 2026 and state law varies. Do not tell a user what their jurisdiction requires — report the arithmetic and the caveats.
Input schema
| Property | Type | Required | Description |
|---|---|---|---|
| subtotal | string | number | no | The pre-tax subtotal. Use this or `items`. |
| items | array | no | Line items, totalled in exact cents. Rounding applies to the TOTAL only. |
| taxRatePercent | number | no | Percentage, e.g. 8.25 — not 0.0825. Supply it whenever it is known: rounding happens after tax, so the rate decides which nickel the total lands on. |
| tender | string | no | Default cash. A card is NEVER rounded — if the user is paying by card, the exact total is the answer and any rounding rule is ignored. |
| rule | string | no | symmetric (default) is the Common Cents Act and Canadian method. always-down is Indiana's rule for cash payments of TAX, not a retail rule. always-up is not law anywhere for retail and is here only to quantify it. none is what a card gets. |
Raw JSON schema
{
"type": "object",
"$schema": "https://json-schema.org/draft/2020-12/schema",
"properties": {
"subtotal": {
"description": "The pre-tax subtotal. Use this or `items`.",
"type": [
"string",
"number"
]
},
"items": {
"description": "Line items, totalled in exact cents. Rounding applies to the TOTAL only.",
"type": "array",
"items": {
"type": "object",
"properties": {
"label": {
"type": "string"
},
"amount": {
"description": "A STRING is read as dollars (\"19.99\"); a NUMBER is read as integer CENTS (1999). A non-integer number is refused rather than guessed at.",
"type": [
"string",
"number"
]
},
"quantity": {
"type": "integer",
"minimum": 1,
"maximum": 9007199254740991
}
},
"required": [
"amount"
]
}
},
"taxRatePercent": {
"description": "Percentage, e.g. 8.25 — not 0.0825. Supply it whenever it is known: rounding happens after tax, so the rate decides which nickel the total lands on.",
"type": "number"
},
"tender": {
"description": "Default cash. A card is NEVER rounded — if the user is paying by card, the exact total is the answer and any rounding rule is ignored.",
"type": "string",
"enum": [
"cash",
"card"
]
},
"rule": {
"description": "symmetric (default) is the Common Cents Act and Canadian method. always-down is Indiana's rule for cash payments of TAX, not a retail rule. always-up is not law anywhere for retail and is here only to quantify it. none is what a card gets.",
"type": "string",
"enum": [
"symmetric",
"always-down",
"always-up",
"none"
]
}
}
}