How it works
An idle-cash calculation measures the opportunity cost of every dollar sitting in checking beyond the working buffer — the gap between what your balance is and what one month of genuine expenses requires. Checking accounts pay near zero by design; savings at current yields pay real money on exactly the same risk-free dollars. The defaults show the pattern most households share: $14,000 in checking against a $4,000 buffer strands $10,000 of dead money, forfeiting $400 annually at a 4% APY and compounding to nearly $4,800 of lost growth across a decade. The figure surprises because it never appears as a line item — there is no statement showing the interest that failed to arrive. The right buffer size resists precision deliberately: one month of expenses absorbs timing mismatches between autopays and paydays, while anything beyond two months of spending belongs in savings or investments unless a known expense looms within weeks. Sweeping need not be heroic either — a recurring payday transfer of whatever exceeded last month's balance automates discipline out of willpower. The only costs are minutes of setup and one extra transfer when an occasional large bill overdraws the buffer, both trivially cheaper than the four-figure decade-long bill the projection table quantifies.
Formula
Idle = checking − required buffer | Annual drag = idle × APY | Horizon cost = idle × ((1 + APY)^years − 1)
Tips
- Size the buffer at one month of expenses; more belongs where it earns.
- Automate a payday sweep of anything above last month's closing balance.
- Keep the buffer itself in checking without guilt — its job is absorbing timing.
- Before big known bills, pre-stage funds back into checking rather than overdrafting.
- Re-run this after windfalls — inheritances and refunds love to hide in checking.
Frequently asked questions
- How much should sit in my checking account?
- Roughly one month of expenses plus any bills due before payday — commonly $3,000–$5,000 for mid-income households. Everything beyond that earns nothing while waiting. The default profile shows $4,000 as the working floor with $10,000 stranded above it, costing $400 yearly at current yields.
- What is the opportunity cost of idle cash?
- Idle dollars times the yield they could earn elsewhere. Ten thousand dollars at a 4% APY forfeits $400 annually and compounds to about $4,802 of missed growth over ten years. It is invisible on statements precisely because it manifests as interest that never arrives.
- Should I move everything out of checking?
- No — keep the buffer that prevents overdrafts and covers billing-timing gaps. Moving everything invites $34 fees that dwarf the yield gained. The strategy is a floor, not an empty account: automate sweeps above the floor and let checking do its job at minimal size.
- Is a high-yield savings account safe for large balances?
- Yes up to FDIC limits — $250,000 per depositor, per institution — identical government backing to any bank deposit. Above those ceilings, split balances across institutions or add Treasury bills, which carry full-faith-and-credit backing with state-tax-free interest and same-week liquidity for larger sums.
- How often should I sweep excess cash to savings?
- Monthly on payday works because it rides existing automation habits. Quarterly suffices for stable balances; sweep immediately after windfalls regardless of calendar. The cadence matters far less than having one — unswept surpluses drift back into spending with remarkable reliability.