Habitual impulses are deceptively expensive because frequency multiplies cost. The example in the question was a $20 impulse item purchased five times per week. Multiply the per-item amount by purchases per week and then by weeks per month (use a 4-week month in this exercise). The arithmetic is simple and instructive: $20 × 5 purchases × 4 weeks = $400 per month. Turning recurring impulses into a monthly line item reframes the decision: $400 a month is unlikely to be a trivial expense in most household budgets. Recognizing the real monthly cost helps you evaluate alternatives — cheaper substitutes, batching treats, or allocating a weekly 'fun money' envelope.

To change the habit, use commitment devices: pre-commit to a weekly spending cap, place a short cooling-off period (e.g., 24 hours) for nonessential purchases, or automate small savings each time you choose not to buy (micro-savings). Another practical tactic is to make impulse purchases slightly harder: remove saved payment card info from mobile apps, delete one-click checkout options, or move tempting apps off your home screen. If you want a behavioral nudge, convert avoided impulse money to a visible goal (put the $400/month into a 'mini vacation' jar) — seeing progress motivates continued restraint. Finally, audit impulses monthly to see whether the habit persists or declines once you treat it as real budget line item rather than invisible small buys.