static vs flexible budget

Imagine you plan a party for exactly 50 guests and budget food costs on that basis. Then 70 people show up. Your original budget said you'd spend a fixed amount for 50 — but of course feeding 70 costs more, so comparing your actual food bill to the 50-guest plan is unfair: it mixes 'you were busier than planned' with 'you overspent per guest.' To judge fairly, you'd redo the budget for 70 guests. That re-done-for-actual-activity budget is the idea behind a flexible budget.

A static (or fixed) budget is set for one planned level of activity and does not change, even if actual volume turns out different. A flexible budget, by contrast, is recalculated for the activity level actually achieved, by flexing the variable costs (those that rise with volume) while holding the fixed costs constant. Suppose a budget assumes 50 units with variable cost 10 each (500) and fixed cost 1,000, total 1,500. If you actually make 70 units, the flexible budget is 70 × 10 + 1,000 = 1,700 — that is the fair benchmark for 70 units, not the original 1,500.

The distinction matters for fair performance evaluation. Comparing actual results against the static budget muddles two very different things: did you do more or less business than planned (a volume difference) and did you control costs at that business level (a spending difference). The flexible budget separates them: the static-budget variance splits into an activity (volume) variance plus a flexible-budget (spending) variance. A common misconception is that beating the static budget means good cost control — you might simply have sold less, which is not the same as spending wisely.