HOW TO USE
How To Calculate Margin Interest & Borrowing Power
1
Enter your account details
| Input | What it means |
|---|---|
| Account equity | Total value of cash + securities in the account (e.g. $10,000) |
| Margin interest rate | Annual rate your broker charges on borrowed money (check your statement — often 6–12%) |
| Leverage | How much your broker will lend against your equity (2× = standard US Reg-T) |
| Maintenance margin | Minimum equity % you must keep (25% in the US) |
2
Set how much to borrow & for how long
1. Drag the “Amount to borrow” slider — the note below shows your maximum borrowing power (equity × leverage).
2. Set the borrowing period (1–60 months) to see the total interest over that time.
3
Read the borrowing cost
The top card shows your total interest cost, broken into daily / monthly / annual amounts. Below it, your total buying power and loan-to-value (LTV).
Example
$10,000 equity · borrow $5,000 · 8.5% · 12 months → $425 in interest
4
Check your margin call risk
This is the part you shouldn’t skip. The “Margin call risk” panel shows:
•How far your portfolio can fallbefore your broker demands more money.
•Minimum equity requiredto stay in the account.
•Arisk meter— green (safe) → yellow (moderate) → red (high risk).
5
See how leverage cuts both ways
The “If your borrowed position moves…” panel shows your return on equity if the portfolio moves −20% to +20%. Notice the losses are just as amplified as the gains — that’s the whole lesson of margin. -20%
large loss -10%
loss +10%
gain +20%
large gain !
Remember before you borrow
•Marginamplifies lossesas much as gains.
•A margin call canforce you to sellat the worst possible moment.
•Interest accruesdaily— the longer you hold, the more it costs.
•Rules vary by broker and jurisdiction — this is an estimate, not a contract
MarginPro
Account & borrowing terms
Standard US Reg-T margin is 2× (50% initial margin). Day-trading accounts can reach 4×.
Typically 25% in the US. Below this, the broker issues a margin call.
Max borrowing power: $10,000
Total borrowing cost
Daily
$1.16
Monthly
$35.42
Annually
$425
Total buying power
$20,000
Equity + borrowable amount at your leverage.
Loan-to-value (LTV)
50%
Your borrowing as % of max power.
Margin call risk
Equity can fall before a margin call
—
Portfolio value decline threshold.
Minimum equity required
—
Maintenance margin on your position.
—
If your borrowed position moves…
-20%
—
-10%
—
+10%
—
+20%
—
Return on your equity after interest, given a portfolio move of that size. Leverage magnifies both gains and losses.
Margin trading is risky. Borrowing to invest amplifies both gains and losses, and a margin call can force you to sell at the worst time. This calculator is for education only — rates, requirements and rules vary by broker and jurisdiction. Consult a financial advisor before trading on margin.
The Day I Learned What “Buying Power” Actually Costs
There’s a little number that appears on your brokerage account if you’ve been trading for a little while. It’s right there, in the upper corner with your cash, and it’s called your buying power. And every time you see it, it’s bigger than the amount of cash you have.
Mine said something like forty thousand dollars, and I had ten in cash at that moment
nd I remember thinking, “well, that’s just free money, right?”
It is not. And the reason why it takes me so long to understand that, and I eventually made this calculator about it, is the topic of this article.
The seduction of borrowing against yourself
Here’s what margin accounts basically are: your broker looks at the stuff you have in your account and lets you borrow money against that (by increasing buying power). Everything sounds great, as when you do that you’re essentially buying more of the same thing with the money you just got, thus increasing the value of the thing you originally bought, which in turn increases profits. It’s just a bigger shovel to dig the same hole, except you don’t actually own the shovel.
What they don’t say on the brochure (or do say in incredibly tiny print) is that you now owe the person who lent you the money some interest, every day, and said interest is compounded against you the same way the daily change to your investment is.
And so I borrowed against my account, of course I did, because I did the math everyone does when first getting exposed to buying power, and only did half of it.
I calculated how much I would have made should the thing I bought went up by 20%, and hadn’t factored in how much I would have lost if it went down by the same percentage.
It went down by 20%
What actually happened (and what I actually wish I’ve known)
It wasn’t much of a shock. It was a calm, boring day, and while I watched my position decrease, slowly, steadily, the meter (as I’ve decided to call the amount owed by a leverage account) was also steadily climbing. Each day’s charge of interest was like tax day, and every new day felt like a surprise bill. Altogether, this debt was quietly bleeding away my account, until the day I came to check on it, realized just how much I actually owed, and was sick with horror. And then I realized there was another, far more terrifying thing that could happen: the margin call.
A margin call is a notice sent by your broker (usually by email!) informing you that your account is now below the minimum percentage of equity required by them, and asking for you to add more money. If you don’t, your broker will sell some of the assets you have with them to pay for the debt and bring your account back to the desirable level. Margin call is something everyone in finance has heard of, is the name of the kind of dramatic scene you can imagine in a movie about Wall Street. But in reality, it’s a cold, bland email, saying that you either need to add more money to your account or they’ll sell your portfolio at whatever price it’ll be at that moment to do so.
That’s the part nobody really explains, I think. In reality, a margin call does not ask you to sell your assets and only consider doing so if you really want to. It doesn’t care about your long term investing strategy and if riding through the wave of the market is what you usually do. A margin call is an automatic event triggered by your broker’s computer, and the only question is whether you’ll have enough money on hand to do anything about it when it happens. And so, I made this calculator because of course I did
What this calculator is for
Let me be very clear: I’m not a financial advisor and in no way affiliated with anyone in the financial industry. What I am is a guy who got educated in the ways of margin accounts the hard way and strongly believes other people should also do it properly before ever considering buying something with a bigger shovel. So I made a calculator for that, a margin borrowing calculator called MarginPro which estimates some of the things I didn’t know about and, in turn, made me lose a lot of money.
When trying to visualize the costs of leverage, this calculator shows you
What exactly you’re paying for: what the amount of interest actually is. How much you can borrow: this is what I didn’t know about, and it turns out your buying power really does have a ceiling, not a floor. When the margin call is coming: this is what I wish to everyone who reads this, a way for them to see exactly how close to insolvency they’re getting without knowing it. How leverage affects everything: there’s a little widget there that shows you what kind of change in portfolio value you’ll see in your return on equity, and since everything is mirrored I really think you should know just how much damage a leveraged position can do when things go sideways
The thing I think I should say here is that
borrowed money changes everything.
It makes you behave differently, not consider as viable options as many things as you would have otherwise, and really, it’s not the kind of thing you can consider in a vacuum. That’s why this calculator is a good idea, for everyone who knows about margin accounts but doesn’t fully know how they work. Used responsibly and without surprise, this tool’s purpose is educational, and it’s here to help you understand that the big friendly number that is your buying power actually comes with strings attached that will affect you in many ways, most of which will be fairly obvious when you actually look at them. And that, I think, is its purpose.
MarginPro is a free margin borrowing calculator. It can estimate the cost of leverage, buying power, margin-call thresholds, and leverage ROI multipliers. It’s designed to be used for educational purposes and is not intended to be financial advice in any capacity. The rates, requirements, and other conditions may vary significantly between jurisdictions and brokerage companies, so please make sure to conduct your own research and consult a professional before engaging in margin trading.