Calculator methodology
The exact rules behind every number.
A complete public statement of how Cairn's child support, spousal support, and tax engine work. Citations to source authority, and the cross-validation discipline against DivorceMate Tools One.
These are the rules the free calculator runs — see your numbers in five minutes.
On this page
What's covered here.
Last reviewed: May 2026 · Ontario family law and the federal support tables. Reviewed against source authority on this date.
Engine · Child support
Child support — Federal Guidelines + Schedule 1
Cairn's child support engine applies the Federal Child Support Guidelines (SOR/97-175 — the federal regulation that sets the tables). The base monthly amount comes from Schedule 1, Ontario column, indexed to the payor's gross annual income (T1 line 15000) and the number of children. For sole-care arrangements (kids living mostly with one parent), this table amount is determinative. The recipient's income does not change the base amount.
Section 7 special expenses — childcare, post-secondary, extraordinary educational and extracurricular expenses, health-related expenses over $100/year not covered by insurance, dental — are added on top and shared between parents in proportion to their gross incomes.
Shared-parenting calculations under section 9 (each parent has 40%+ of the time) and split-custody under section 8 are not handled in V1. Both are on the V2 plan.
Engine · Spousal support
Spousal support — Spousal Support Advisory Guidelines (SSAG)
Cairn's spousal support engine applies the Spousal Support Advisory Guidelines (2008, revised 2017). Two formulas:
Without-children formula. Amount range = (1.5% × years × gross-income difference) up to (2% × years × gross-income difference), capped at 50% of the gross-income difference. Duration = 0.5–1.0 × years. The Rule of 65 (years + recipient's age at separation ≥ 65, with at least 5 years of relationship) flips duration to indefinite.
With-children formula. Operates on individual net disposable income (INDI — what each parent has left after tax, CPP/EI, and notional child support). The recipient's INDI is targeted at a percentage of combined INDI on a sliding scale that depends on years of relationship and number of children. Spousal support is the amount that closes the gap between the recipient's actual INDI and the target.
Cairn outputs the low / mid / high amount and the SSAG duration range. The mid is shown as the headline number; low and high are visible underneath. We don't pretend a single point estimate is the answer — the Guidelines explicitly produce a range.
Engine · Tax
Ontario tax engine — federal + provincial + CPP + EI
The Cairn tax engine implements current-year federal and Ontario provincial tax brackets, the Basic Personal Amount (federal and Ontario), the eligible-dependant credit, the Ontario surtax, CPP base contributions up to the Year's Maximum Pensionable Earnings (YMPE — the CRA-set cap on first-tier CPP contributions), CPP2 (the second-tier contribution introduced 2024, applied between the YMPE and the Year's Additional Maximum Pensionable Earnings or YAMPE), and EI on insurable earnings up to the maximum.
Spousal support is treated as deductible to the payor (line 22000) and taxable to the recipient (line 12800). Child support is neither deductible nor taxable.
Tax tables are updated annually. The current set is sourced from CRA T4032ON, and we maintain an internal runbook for the annual roll-forward.
Engine · Source deductions
T1213 source-deduction view (spousal support payors)
For payors of spousal support, Cairn shows two paycheque scenarios. Default withholding: tax is withheld on gross pay ignoring the spousal support deduction; the deduction returns as a year-end refund. T1213-approved: tax is withheld on gross pay minus the annualized spousal support deduction; cash stays with the payor each pay period.
The Cairn calculator does not file T1213 for you — it shows the gap and explains how. Filing the form, attaching proof, and waiting for CRA's Letter of Authority to your employer is your responsibility. The /calculator/t1213-source-deduction page has a 7-step filing guide.
Sources
Engine · Validation
Cross-validation against DivorceMate Tools One
DivorceMate Tools One is the SSAG/CST engine licensed by most Ontario family lawyers and used in court. We do not license DivorceMate. Instead, every release of the Cairn engine is cross-validated against DivorceMate output across a representative scenario set before merge — payor income from $40K to $250K, recipient income from $0 to $150K, 0–4 children, relationship lengths from 1 to 30 years, and edge cases including the Rule of 65 boundary and CPP2 onset.
Tight numerical regression fixtures gate every release. The cross-validation discipline was codified after an SSAG calibration error in May 2026 shipped wrong numbers through three calculator releases before being caught. Every release since runs the DivorceMate parity check before merge.
Sources
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