How we calculate

Every tool’s formula, where it comes from, and where we made a judgement call.

Budget

50/30/20: needs = 50%, wants = 30%, savings and extra debt payments = 20% of monthly take-home pay (Warren and Warren Tyagi, All Your Worth). Zero-based: take-home pay minus every assigned line; the goal is $0.

Debt payoff

Monthly simulation. Each month: interest = balance × APR ÷ 12 on each debt; every minimum is paid; all remaining money goes to the target debt (smallest balance for snowball, highest APR for avalanche; ties broken by the other rule); a paid-off debt’s minimum rolls forward. One debt: n = −ln(1 − B·i ÷ P) ÷ ln(1 + i), with i = APR ÷ 1,200; payment for a deadline P = B·i ÷ (1 − (1 + i)⁻ⁿ). Real cards charge daily interest, so statements differ slightly.

Savings

Monthly rate = (1 + APY)^1/12 − 1. Months to goal: add a month of interest then the deposit until the balance reaches the goal. Monthly amount for a deadline: (goal − saved × (1 + r)ⁿ) × r ÷ ((1 + r)ⁿ − 1). Emergency fund: monthly essentials × months (3–6 is the usual guidance). Challenges: 52-week = step × (1 + 2 + … + 52) = 1,378 × step; 100-envelope = 5,050 × step; 26 paydays = 2 × step × (1 + … + 26) = 702 × step.

Grocery budget

Sum of the USDA monthly cost for each person by age and sex (August 2026, U.S. average), then the USDA household-size adjustment: +20% (1 person), +10% (2), +5% (3), 0 (4), −5% (5–6), −10% (7+). Weekly = monthly ÷ 4.333, as the USDA does.

Paint and wallpaper

Wall area = 2 × (length + width) × height − 21 sq ft per door − 15 sq ft per window (a 3 × 7 ft door and 3 × 5 ft window, our assumption). Gallons = area × coats ÷ coverage; default 350 sq ft per gallon, the low end of Sherwin-Williams’ 350–400. Rounded up to quarts; three quarts become a gallon. Wallpaper rolls = wall area ÷ (roll area × (1 − waste)); default 56 sq ft and 15%.

Rug size

Living room: the largest standard size that fits the room less 18 in on every side (designers’ range 12–24 in); with a sofa, the rug must be at least sofa length + 12 in (Wayfair: 6 in past each end). Dining: the smallest standard size covering the table plus 24 in on every side (Wayfair: 24–36 in). Bed, lower two-thirds: queen 8 × 10, king 9 × 12 (Wayfair); twin 5 × 8, full 6 × 9 and California king 9 × 12 (our extension). Whole bed: bed + 24 in at the sides and foot, rounded up to a standard size. Standard sizes: 2 × 3 to 12 × 15 ft.

Chore chart

Ages 2–4 from Intermountain Health; ages 5–12 from the American Academy of Pediatrics (HealthyChildren.org), whose bands build on each other. Lists lightly reworded; the chart shows up to eight chores per child.

Water intake

National Academies Adequate Intakes (2005): total water and the part from drinks, by age and sex; 1 liter = 33.814 US fl oz; 1 cup = 8 oz. Breastfeeding drinks figure 3.1 L (about 13 cups). The “half your weight in ounces” comparison is a popular rule, shown for reference only.

Sleep

Hours by age from the AASM pediatric consensus (2016) and the AASM/SRS adult statement (7 or more); upper figures for adults from the National Sleep Foundation (7–9; 7–8 over 65). Bedtime = wake-up − hours − 15 minutes to fall asleep (our assumption). Cycle times count in 90-minute cycles, an average.

Steps to miles

Step length = height × 0.413 (women) or × 0.415 (men), a pedometer rule of thumb, or your measured step. Steps per mile = 63,360 in ÷ step length; 1 mile = 1.609344 km.

What we don’t build

No calorie, BMI, weight-loss, diet or supplement-dose tools. Those need a doctor or dietitian who knows you.