How to calculate it yourself
Total cost = gallons × price per gallon. For multi-room jobs, calculate gallons for each room separately, sum them, then round up to whole gallons (or to a 5-gallon multiple if buying buckets).
Common scenarios
Single bedroom (2 gallons)
Mid-tier paint: 2 × $40 = $80. With 20% coupon: $64. Add ~$30 for primer if needed.
Whole interior (28 gallons)
Buy 5-gallon buckets: 6 × $180 = $1,080. Significant savings vs. $1,400+ in 1-gallon cans.