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Economics

The Shutdown Decision & Sunk Costs

The P ≥ AVC Rule, Fixed vs. Sunk Costs, and Why Losses Don't Always Mean Stop — A TLDR Primer

When your economics exam asks whether a firm should keep producing or shut down, most students freeze — or worse, guess wrong because they confuse sunk costs with costs that actually matter. This concise guide cuts straight to the logic behind one of the trickiest decisions in introductory microeconomics.

**The Shutdown Decision & Sunk Costs** walks you through every cost category a firm faces in the short run — fixed, variable, sunk, and avoidable — and shows you exactly which ones belong in the shutdown calculation and which ones should be ignored. From there it derives the P ≥ AVC shutdown rule step by step, in plain English and with worked numbers, so the logic sticks rather than just the formula. A full walkthrough of the MC, AVC, and ATC cost curves shows you where the shutdown point sits on the graph and how it generates the firm's short-run supply curve. The final section distinguishes a temporary shutdown from permanent exit and applies both to real industries — airlines, restaurants, and oil drilling — so abstract theory connects to headlines you've actually seen.

Written for high school economics students, AP Micro test-takers, and early college students facing their first microeconomics unit, this guide is short by design. No filler, no detours through material you don't need right now. Every section leads with the one thing you must understand, then unpacks it with examples and corrects the misconceptions that trip students up most.

If the shutdown rule and sunk cost fallacy are on your next exam, start here.

What you'll learn
  • Distinguish fixed, variable, sunk, and avoidable costs and identify each in real scenarios.
  • Apply the short-run shutdown rule: produce if price covers average variable cost, otherwise shut down.
  • Explain why sunk costs are irrelevant to forward-looking decisions and recognize the sunk cost fallacy.
  • Differentiate the short-run shutdown decision from the long-run exit decision.
  • Use marginal cost, average variable cost, and average total cost curves to find the shutdown point and the firm's short-run supply curve.
What's inside
  1. 1. Costs in the Short Run: Fixed, Variable, Sunk, and Avoidable
    Defines the cost categories a firm faces and clarifies which ones matter for the shutdown decision.
  2. 2. The Shutdown Rule: Price, AVC, and Why a Firm Keeps the Lights On at a Loss
    Derives and explains the rule that a firm should keep producing in the short run if price covers average variable cost.
  3. 3. Sunk Costs and the Sunk Cost Fallacy
    Explains why sunk costs should be ignored in any forward-looking decision and walks through common student traps.
  4. 4. Working the Graph: MC, AVC, ATC, and the Firm's Short-Run Supply Curve
    Walks through the standard cost-curve diagram, locates the shutdown point, and shows how it generates the supply curve.
  5. 5. Shutdown vs. Exit: Short Run, Long Run, and Real-World Examples
    Distinguishes the temporary shutdown decision from permanent exit and applies both to real industries like airlines, restaurants, and oil drilling.
Published by Solid State Press
The Shutdown Decision & Sunk Costs cover
TLDR STUDY GUIDES

The Shutdown Decision & Sunk Costs

The P ≥ AVC Rule, Fixed vs. Sunk Costs, and Why Losses Don't Always Mean Stop — A TLDR Primer
Solid State Press

The Shutdown Decision & Sunk Costs

The P ≥ AVC Rule, Fixed vs. Sunk Costs, and Why Losses Don't Always Mean Stop — A TLDR Primer

Copyright © 2026 Solid State Press.

Published by Solid State Press.

All rights reserved. No part of this book may be reproduced or transmitted in any form without prior written permission, except for brief quotations in critical reviews and educational use.

Part of the TLDR Study Guides series.

Contents

  1. 1 Costs in the Short Run: Fixed, Variable, Sunk, and Avoidable
  2. 2 The Shutdown Rule: Price, AVC, and Why a Firm Keeps the Lights On at a Loss
  3. 3 Sunk Costs and the Sunk Cost Fallacy
  4. 4 Working the Graph: MC, AVC, ATC, and the Firm's Short-Run Supply Curve
  5. 5 Shutdown vs. Exit: Short Run, Long Run, and Real-World Examples
Chapter 1

Costs in the Short Run: Fixed, Variable, Sunk, and Avoidable

Every business decision boils down to one question: which costs can I actually do something about? To answer that, you need a precise vocabulary — four terms that economists use to slice up a firm's total cost bill.

Fixed costs are costs that do not change with the quantity of output the firm produces. Whether the firm makes 1 unit or 10,000 units, fixed costs stay the same. A bakery's monthly rent is a fixed cost. So is the salary of a full-time manager hired on an annual contract, or the premium on a commercial insurance policy. Notice that "fixed" does not mean small — it just means unresponsive to output.

Variable costs are the mirror image: they rise and fall with output. The bakery's flour, eggs, and butter are variable costs. So are the wages of hourly workers hired by the shift. When the bakery bakes more bread, it buys more flour. When it shuts the ovens for the weekend, the flour bill drops to zero. That last sentence is worth lingering on: variable costs can fall all the way to zero if output falls to zero.

The short run is any time horizon short enough that at least one input is fixed — typically capital (equipment, a leased building, machinery). In the short run, the bakery cannot renegotiate its lease or sell its industrial ovens on short notice. The long run is the planning horizon long enough that all inputs are adjustable. This book is almost entirely about short-run decisions, because that is where the shutdown question lives. In the long run, a firm that is losing money just exits — the harder question is what to do right now, before any contracts can be renegotiated.

Now for the two concepts that students most often mix up.

A sunk cost is a cost that has already been paid and cannot be recovered, regardless of what the firm does next. The bakery owner paid a $5,000 non-refundable deposit on a custom bread-slicer six months ago. That $5,000 is gone whether the bakery operates tomorrow or not. It is sunk. Sunk costs are a subset of fixed costs — they are fixed costs that are also irrecoverable.

An avoidable cost (sometimes called an escapable cost) is any cost the firm can eliminate by changing its behavior. If the bakery stops operating, it stops buying flour — so the flour cost is avoidable. If the bakery can sublease its kitchen space to another tenant, the rent becomes avoidable too. The key test: Does this cost disappear if I shut down? If yes, it is avoidable. If no, it is either sunk or otherwise inescapable.

About This Book

If you are a high school student working through AP Microeconomics, a college freshman in Principles of Microeconomics, or anyone who has stared at a firm cost curves graph and felt lost, this book is for you. It is also useful for tutors prepping a session on producer theory and parents helping a student review before an exam.

This is a focused microeconomics study guide for high school students and early undergraduates covering the short run shutdown rule, the average variable cost shutdown point, and how to read the MC, AVC, and ATC curves that appear on nearly every micro exam. It tackles the sunk cost fallacy explained plainly alongside fixed, variable, and sunk costs — the distinctions that trip up students when they ask when should a firm shut down in economics. Short by design, no filler.

Read straight through in order, work every worked example as you reach it, then test yourself on the problem set at the end.

Keep reading

You've read the first half of Chapter 1. The complete book covers 5 chapters in roughly fifteen pages — readable in one sitting.

Coming soon to Amazon