Three terms that sound like an MBA exam but quietly control what you see on your investment app: market breadth, index weighting and free float. Here’s what they mean, and why your Top 40 ETF sometimes behaves like a fan club for a few big names.
By The Money Maze | Listen to the episode: Spotify
“The JSE closed 2% higher today.” Sounds like a party. But was everyone dancing, or were three okes carrying the whole dance floor? That’s the question these three concepts answer.
What is market breadth?
Market breadth measures how many shares are joining in when the market moves.
The basic version compares advancing shares with declining shares. If 35 of the Top 40 are up, breadth is strong. If only five are up and 35 are sulking in the red, breadth is weak, even if the index closed higher because the five winners were giants.
It’s a braai where one guy brought steak and everyone else brought pap. Technically there’s food. It’s not balanced.
Why breadth matters
Breadth tells you whether a rally has legs. A broad rally, with most shares rising, suggests investors are confident across the market. A narrow rally, carried by a few heavyweights, is more fragile: if those few stumble, there’s nothing underneath.
Common breadth measures:
- Advance-decline line: a running total of advancing minus declining shares.
- New highs vs new lows: how many shares hit 52-week highs versus lows.
- Percentage above a moving average: how many shares trade above, say, their 200-day average.
Breadth on the JSE is a special case
The JSE has a quirk that makes breadth extra useful. Many of its biggest companies earn most of their money offshore: global internet stakes, luxury goods, international mining. Meanwhile, many smaller companies depend on the South African economy. So the Top 40 can rise because of Chinese tech or gold prices while local banks, retailers and builders struggle. Breadth helps you separate “global giants had a good day” from “South Africa Inc. had a good day”. They’re very different stories, and they matter differently depending on what you own.
What is index weighting?
Index weighting decides how much each share influences the index. Most major indices, including the FTSE/JSE Top 40, use market-cap weighting adjusted for free float. Bigger companies carry more weight.
Picture a tug-of-war team with Dwayne “The Rock” Johnson and nine Grade 5 prefects. Guess who’s doing the pulling.
On the JSE, a handful of giants such as Naspers, Prosus, the big miners and Richemont have carried very large weights at different times. When those few move, the index moves, whatever the other 30-odd companies are doing. That’s why capped indices exist, putting a ceiling on any single share’s weight.
Other approaches:
- Equal weighting: everyone gets the same size chop, even the guy who only brought ice.
- Price weighting: higher share price, more influence. Old-school, like the US Dow. Price doesn’t equal size, which is why it’s rare.
- Fundamental weighting: based on earnings, dividends or other measures.
If you own a market-cap-weighted Top 40 ETF, you’re making a big bet on the biggest names. Check the top holdings so you’re not surprised.
What is free float?
Free float is the share of a company’s stock that’s actually available to trade. Not all shares in issue are on the market. Some are held by founders, controlling shareholders, governments or strategic partners who don’t plan to sell.
Example: a company has 1 billion shares. The founder holds 400 million and a strategic partner holds 200 million. The free float is the remaining 400 million, or 40%.
Why free float matters
- Index weight: FTSE/JSE indices weight companies by their free-float market value, not total value. A giant with a small free float gets a smaller weight than its size suggests. Free float figures are updated at the quarterly reviews.
- Volatility: with fewer shares available, small trades can move the price a lot. When a small-cap swings like a DJ at a wedding after too many tequilas, check the free float.
- Liquidity: thin free float often means wider bid-offer spreads, so you pay more to get in and out.
Putting it together: a real-world reading
Imagine the Top 40 rises 1.5% in a day. You check breadth: only 12 of 40 shares rose. Two heavily weighted shares jumped 5% on offshore news. The rest drifted lower.
What does that tell you? The headline number came from a few giants, not from broad confidence in South African companies. If you own a Top 40 tracker, you benefited. If you own mid-caps or local industrials, your portfolio may have gone backwards on a “good” day.
Why you should care
- Breadth: tells you whether a rally is broad or top-heavy.
- Weighting: explains why some shares dominate your ETF.
- Free float: tells you how liquid a share is and whether you’re walking into a volatility party.
Stokvel Starter Pack
- Market breadth: how many shares join a market move.
- Advance-decline line: a running count of risers minus fallers.
- Market-cap weighting: bigger companies count more.
- Capped index: a ceiling on any single share’s weight.
- Free float: shares actually available to trade.
- Thin free float: fewer shares trading, bigger price swings.
Takeaway: Don’t judge the market by the headline number alone. Check who did the heavy lifting, how much they weigh, and how many shares are really floating.
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More from The Money Maze
- JSE Index Changes and Rebalancing: How the Top 40 Is Picked
- Trading Volume, Bid/Ask Spread and Liquidity Explained
- ETF vs Unit Trusts, NAV and TER Explained
- Rebalancing, Asset Allocation and Diversification Explained
Sources
- FTSE/JSE Africa Index Series ground rules v9.1, JSE and FTSE Russell.
- FTSE/JSE Africa Index Series ground rules v8.8, JSE and FTSE Russell, Mar 2025.
This is not financial advice.
