What do a, b, c, and d represent?

In the linear model, a is the quantity supplied at price zero (supply intercept), b is the rate at which supply increases with price (supply slope), c is the quantity demanded at price zero (demand intercept), and d is the rate at which demand falls with price (demand slope). All four must be positive for a standard market model.

What if b + d equals zero?

The denominator b + d cannot be zero in a standard model. If both slopes are zero, supply and demand are both flat (perfectly inelastic price response), so no unique equilibrium exists. The calculator will show an error in this case.

What if the equilibrium price is negative?

A negative equilibrium price means the supply intercept exceeds the demand intercept (a > c). In practice this indicates a surplus at all positive prices; markets usually have a price floor of zero. The calculator will display the mathematical result and note that the price is below zero.

Can I model non-linear supply or demand curves?

This tool assumes linear (straight-line) functions, which is the standard first-year economics approximation. Non-linear curves require numerical methods and are beyond the scope of this calculator.

How accurate are the results?

Results are computed with JavaScript floating-point arithmetic and rounded to four decimal places for display. For typical textbook values the results are exact. Extremely large or small coefficients may produce minor rounding differences.

Equilibrium Price Calculator

Name: Equilibrium Price Calculator
Availability: InStock
Author: Nham Vu

Enter linear supply and demand functions to instantly find the market equilibrium price and quantity, with a chart.

Supply Function — Qs = a + bP

Intercept (a)

Qs when P = 0

Slope (b)

Change in Qs per unit P

Demand Function — Qd = c − dP

Intercept (c)

Qd when P = 0

Slope (d)

Decrease in Qd per unit P

Solving formula

P* = (c − a) / (b + d)

Q* = a + b × P*

Supply & Demand Chart

Enter values and click Calculate to view the chart.

Summary

Enter linear supply and demand functions to instantly find the market equilibrium price and quantity, with a chart.

How it works

Enter the supply intercept (a) — the quantity supplied when price is zero.
Enter the supply slope (b) — how much quantity supplied increases per unit price rise.
Enter the demand intercept (c) — the quantity demanded when price is zero.
Enter the demand slope (d) — how much quantity demanded falls per unit price rise.
Click "Calculate" to solve for equilibrium price P* = (c - a) / (b + d) and quantity Q* = a + b * P*.
Read the result and inspect the supply-demand chart to visualize the intersection.

Use cases

Homework and exam preparation for introductory microeconomics courses.
Quickly verify hand-calculated supply-demand intersections.
Visualize how shifts in supply or demand change the equilibrium point.
Teaching tool for illustrating the law of supply and demand graphically.

Frequently Asked Questions

Last updated: 2026-06-11 · Reviewed by Nham Vu