Don't Size Your Emergency Fund Only in Months: Working Backward from the Worst-Case Unexpected Expense

Good morning, this is まさきん.

Our family recently rethought how we size our emergency fund. We set aside the usual yardstick of “so many months of living expenses” for a while. I want to write down how I sorted out my thinking.

This is not a post recommending investments. It is a note, from the perspective of a financial planner (FP), on how to measure a household’s safety net. The final call depends on your own family’s situation.

※This information is current as of publication. Please check the official websites for the latest conditions.

We planned to keep 12 months’ worth

At first, our family planned to keep an emergency fund of “12 months of living expenses.” We are a dual-income household with four kids. So I thought having a thick cushion would give us peace of mind.

But when I thought about it again, something bothered me. What exactly was the 12-month figure meant to protect against? I could not explain it.

In the end, we decided to set the amount not by “months of living expenses” but by “the realistic worst-case unexpected expense.” We compared several options and chose the smallest one. It was a downward revision from our original plan.

What the common guidelines say

First, let me check the guidelines you often hear. Here is what I could confirm from public and neutral sources.

The Japan FP Association page gives a guideline with a wide range. From 3 months to 1 year is quite a spread. On the other hand, I could not find a finer distinction between employees and the self-employed in any public materials.

So the bottom row is probably best taken as “what people often say.” Personally, I think the width of the range is itself a hint. It may mean the right number differs from household to household.

What “months of expenses” protects against

The idea of so many months of living expenses assumes a situation where income stops. For example, when you lose your job or cannot work because of illness. The image is bridging your living costs during that time with cash on hand.

J-FLEC’s lecture materials divide money into three groups: “money needed for daily life,” “money you plan to use in the near future,” and “money you do not plan to use for the time being.” I think an emergency fund falls under the safety net that protects daily life.

But income stopping is not the only thing that can knock a household budget off balance. The water heater breaks. Several car repairs pile up. Sudden expenses on the “money going out” side can shake the budget just as much.

Households better off thinking in months

For households with a high risk of income stopping, I think thinking in months of expenses is a good fit. For example, households like these.

• Self-employed or freelance, with income that swings a lot from month to month
• A large share of pay comes from commission or performance-based pay
• Planning to change jobs or go independent soon
• Only one source of income supports the household

For these households, it is hard to predict how long income might drop to zero. So the yardstick of “how many months can we last” works well. For the self-employed, some of the public benefits available to employees are not available. You could say the reasons for keeping a thicker cushion are clear.

What happens with two income sources

On the other hand, the situation is a little different for dual-income households with two sources of income. The chance that both people lose their income at the same time is presumably lower than for one person. Even if one stops, the other income remains.

Also, if you are an employee, there are public benefits when you take time off or lose your job. The period when income drops completely to zero may be shorter than you imagine.

Seen this way, the need to hold many months of full living expenses fades. Instead, isn’t it the sudden large expenses you should really be worried about?

How much public programs can cap

Here, let me check what the public programs actually offer. Medical costs, time off and unemployment all have caps or benefit systems.

The Ministry of Health, Labour and Welfare page also gave a concrete example. It is for someone under 70 with an annual income of about 3.7 million yen to about 5.1 million yen, under the system from August 2026. Even for treatment costing 1 million yen, the out-of-pocket amount is said to be capped at about 93,000 yen.

Laid out as numbers it looks dry, but I feel this is an important point. Neither medical costs nor lost income are unlimited. They give you a ceiling to work with when you think about “how bad can it get.”

However, benefits come with waiting periods and reviews. It takes time before the money arrives. You still need cash to bridge that “gap.” I think it matters to look at both the program caps and the gap.

Looking at sudden expenses by worst-case amount

So how do you estimate sudden expenses? Here, I will call “the realistic worst case among possible expenses” the worst-case amount.

The key is not to assume “everything happens at once.” If you add it all up, you end up with an amount larger than many months of expenses. It may be more realistic to decide on a combination, such as “one big thing and one medium thing happening together.”

Steps to work backward from the worst-case amount

Here is the flow of thinking, written as steps.

1. List possible sudden expenses. For each category in the table above, write down the equipment in your home and events that could happen.
2. Estimate the worst-case amount. Put a rough figure on each one. Also decide which combinations could happen at the same time.
3. Subtract what public programs cover. Think about medical costs in terms of the high-cost medical expense cap. For time off and unemployment, assume the benefits and keep only the gap period.
4. Hold the rest in cash. Whatever is left after subtracting is what you keep in cash as your emergency fund.
5. Decide when to review. Review it when your family structure or work situation changes, or after a big replacement purchase.

I think step 3, the “subtract,” is the heart of this method. You may not need to hold your own cash for everything that public programs already cover. That said, the details of the programs vary depending on your insurance and employer. Please check your own conditions with your insurer or your employer’s guidance.

Why we went with the smallest option

Our family lined up several options and compared them. Then we settled on the smallest one.

The reason is that, right now, there was something we wanted to prioritize over a thicker emergency fund: improving our monthly cash flow. Specifically, we decided to move ahead with reviewing fixed costs and loan repayments first.

Even with a thicker emergency fund, the sense of security does not last if the monthly budget stays tight. Conversely, if you have more room each month, you can refill the fund faster after using it. Personally, I feel this does more to strengthen the household’s underlying finances.

The nice thing about reviewing fixed costs is that once you do it, the effect keeps going. For example, your phone bill may be one of the easier items to tackle.

Rules to set if you keep a thinner fund

Keeping a thinner emergency fund carries a clear risk. When it runs short, it is easy to drift toward borrowing. So if you decide on an amount, you want to set rules at the same time.

I think the first two are especially important. Installment payments and revolving credit let you pay even when you have little cash on hand. They are convenient, but they come with fees. The balance also tends to be harder to see.

J-FLEC’s materials also say investing should be done with spare money. If you fill the gap from cutting your emergency fund with borrowing, you have lost the plot. If you want to tidy up how you use cards, the idea of consolidating to one main card may also be helpful.

Who this suits and who it does not

I do not think this backward-calculation method fits every household. Roughly speaking, it looks like this.

If you fall on the “may not suit” side, thinking in months of expenses may give you more peace of mind. Rather than one being right, I think it is something to choose based on the shape of your household.

Common pitfalls

The backward-calculation method also has a few pitfalls.

• Estimates get too optimistic. If the desire to keep the amount small comes first, you tend to underestimate the worst case.
• Over-relying on programs. Even with caps and benefits, you need cash until the money arrives.
• Putting off refilling. If you leave it depleted, you cannot withstand the next sudden expense.
• Forgetting to review. When your family grows or your work changes, the assumptions break down.

I think anyone can fall into the first one especially easily. Estimating a little on the high side may be just right.

What to check before you start

• How many income sources the household has, and how likely they are to stop at the same time
• The type of health insurance you have, and whether it offers the injury and sickness allowance
• What the high-cost medical expense cap is for your age and income bracket
• Whether you are enrolled in employment insurance
• How old your appliances, car and home equipment are
• Whether your family can agree not to use card installment payments or revolving credit
• How many months it would take to refill the emergency fund after using it

Frequently asked questions

Q. So how many months of living expenses is the right answer?

I think the right answer differs for each household. The Japan FP Association page gives a range of 3 months to 1 year of living expenses. Where you land within that range will likely depend on how stable your income is and your family structure.

Q. If we are a dual-income household, can we keep a smaller emergency fund?

You may be able to. However, if both of you work in the same industry, you could both be affected at the same time. Check how “separate” your income sources really are.

Q. Can I use this method if I am self-employed?

It is not impossible, but you may want to think carefully. In many cases, the injury and sickness allowance and the basic unemployment allowance for employees are not available as they are. If you are self-employed, I think it is safer to use the months-of-expenses approach as your base.

Q. If there is a high-cost medical expense benefit, do I not need to prepare for medical costs?

I do not think you can go that far. Even with a cap, you still pay out of pocket up to that cap. Costs not covered by insurance and transportation to appointments also add up separately.

Q. Can I invest my emergency fund through the new NISA?

Personally, I think the basic approach is to keep an emergency fund in cash, which does not fluctuate in value. When you suddenly need it, it may have dropped in value. Whether to invest it is your own decision.

Q. How often should I review it?

Rather than a fixed schedule, I think the time to review is when your household’s assumptions change. For example, when your family grows or your work situation changes. After a big replacement purchase is also a good time. Lining it up with an annual review of your overall household budget seems like a good idea too.

Q. What if I want advice on my own household’s situation?

For individual decisions, I think it is best to consult a specialist such as an FP. You can check the detailed conditions of public programs at your health insurer or at the Hello Work office. This post is about organizing the way of thinking, so please do not apply it as is.

Official sources I referred to

• Guidelines for the costs of major life events (Japan FP Association) (Japanese)
• Lecture material: Life planning and household finance (J-FLEC, Japan Financial Literacy and Education Corporation) (Japanese)
• For those using the high-cost medical expense benefit (Ministry of Health, Labour and Welfare) (Japanese)
• Injury and sickness allowance (Kyokai Kenpo) (Japanese)
• About the basic allowance (Hello Work Internet Service) (Japanese)
• Specific procedures for employment insurance (Hello Work Internet Service) (Japanese)

Summary

Emergency funds are often talked about in terms of months of living expenses. But for households with two income sources, you can also work backward from the realistic worst-case unexpected expense. If you subtract the caps from public programs, the cash you need may turn out smaller than you thought.

Our family revised our original plan of 12 months and chose the smallest of several options. In exchange, we are prioritizing improving our monthly cash flow. Not going back to revolving credit and refilling the fund after using it go hand in hand with that.

If you want to start with reviewing fixed costs, how to get started reviewing fixed costs may be helpful. I wrote about the refund process for medical costs in my post about paying our baby’s medical bills upfront.

The final amount will depend on your family’s situation. As a first step, how about writing down the ages of the equipment in your home?

Lowering fixed costs like your phone bill may make refilling your emergency fund a little easier too.

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