Aggregate Supply and the 45-Degree Line
Class 12 Macro Economics — Where does national income come from, and how do we know if the economy is in balance?
What is Aggregate Supply?
Aggregate = Total. Supply = What is produced. Put them together and you get the total value of goods and services produced in the economy.
AS = National Income (Y)
Here's the key insight: the money received from selling everything produced flows back to households as factor incomes — wages, rent, interest, and profit.
So every rupee of output (AS) becomes a rupee of income (Y) for someone. That's why AS = Y always holds.
Production side
Firms produce goods worth ₹Y
Income side
Households earn income worth ₹Y
Think of a family business:Rahu runs a small furniture workshop in his garage. Last year he made and sold tables worth ₹4 lakh. That ₹4 lakh was spent on wood, paid as rent for his workshop space, and kept as his own profit. Every rupee of output became someone's income. AS = Y = ₹4 lakh.
Components of Aggregate Supply
Total national income (Y) has two uses: you either spend it (Consumption) or save it (Saving). There is no third option.
Two-Sector Identity
Income Allocation Bar — What happens to each ₹ of income?
What this shows: At income ₹200 cr, households consume ₹200 cr and save ₹0 cr. At exactly ₹200 cr, people spend all they earn — saving is zero. This is the break-even point.
AS Schedule and the 45-Degree Line
The AS schedule shows how much output (income) is produced at each level, and how it splits between consumption and saving.
| Income (Y) (₹ cr) | Consumption (C) (₹ cr) | Saving (S) (₹ cr) | AS = Y (₹ cr) |
|---|---|---|---|
| 0 | 40 | -40 | 0 |
| 100 | 120 | -20 | 100 |
| 200 | 200 | 0 | 200 |
| 300 | 280 | 20 | 300 |
| 400 | 360 | 40 | 400 |
| 500 | 440 | 60 | 500 |
| 600 | 520 | 80 | 600 |
45-Degree Line Explorer
Reading the 45° Chart
The 45° line represents AS = Y — every point on it shows where output (supply) equals income. The C curve starts at ₹40 cr (autonomous consumption) and rises slower than Y.
When C is below the 45° line, the vertical gap is positive saving (income exceeds spending). When C is above the 45° line, that gap is dissaving (spending exceeds income).
Significance of the 45° Line
Why the 45° Line Matters
Every point on the 45° line satisfies Y = C + S. It is the line of reference — it tells us exactly where aggregate supply equals aggregate income at every level. Without it, we would not be able to see whether an economy has a surplus (saving) or deficit (dissaving).
Points on the 45° Line — click any point
Tap any point on the 45° line to see the values and meaning.
45°
The reference line where Y = AS = C + S
C < Y
Above break-even: positive saving (gap below 45° line)
C > Y
Below break-even: dissaving (gap above 45° line)
Summary
Key Takeaways
- Aggregate Supply (AS) = total value of final goods and services produced = National Income (Y).
- The two-sector identity is Y = AS = C + S. Every rupee of output becomes either consumption or saving.
- The AS schedule shows the same data as the income column — AS = Y at every level, split into C and S.
- The 45° line is a reference line where every point satisfies Y = C + S. It helps visualise whether an economy is saving (C below the line) or dissaving (C above the line).
- The break-even point (Y = 200 in our example) is where C = Y and S = 0. Below it, people dissave; above it, they save.