You and your spouse sit down on Sunday morning with coffee. One of you pulls up a Chase checking account in dollars. The other opens a Mitsubishi UFJ app showing yen. You also have a Fidelity 401(k), a NISA account in Tokyo, a Rakuten credit card bill, and a Vanguard brokerage. Someone asks the simple question: how much are we actually worth this month?
And nobody has a clean answer.
If you're a dual-income couple straddling the US and Japan, you've probably felt this. The yen dropped from 110 to 155 against the dollar in three years. That single FX move can swing your net worth by 20-30% without you buying or selling anything. I've watched couples assume they were saving $4,000/month only to realize that when converted properly, their real USD-denominated net worth went down for six months straight because their yen-side assets were getting eaten by FX.
This article walks through how I built a multi-currency net worth dashboard that actually works for cross-border households — the account structure, the FX handling, the monthly review ritual, and the mistakes I made before landing on something sustainable.
The hidden cost of not tracking cross-border finances properly
Let me put numbers on it.
A couple I'll call Ken and Sarah — Ken works remote for a Tokyo firm paid in yen, Sarah works for a US tech company paid in USD — told me they were "saving around $80,000/year combined." When we actually reconstructed their 2022-2024 net worth in a single base currency:
- Their yen-side savings grew from ¥8M to ¥11M (looks great)
- But in USD terms, those yen assets went from ~$72k to ~$71k
- Their actual USD-measured savings rate was closer to $52k/year, not $80k
- They'd been over-contributing to a Japanese savings account that was structurally losing purchasing power relative to their US retirement goals
That's a $28k/year blind spot. Over a decade of compounding, that's the difference between retiring at 55 and retiring at 62.
The other cost is softer but real: fights. Every couple I know with cross-border finances has had the "wait, how much do we have?" argument. Not because anyone's hiding money, but because nobody has a single source of truth.
Why generic net worth trackers break for US-Japan couples
I tried them all before building my own. Here's where each one fell apart:
| Tool | Why it broke |
|---|---|
| Mint / Rocket Money | USD-only. No Japanese bank integration. |
| Monarch Money | Great for US, but cannot pull SBI, MUFG, Rakuten, or NISA accounts |
| Moneytree (Japan) | Excellent for JP accounts, no US coverage, UI in Japanese only |
| YNAB | Budgeting-first, not net-worth-first. Single currency. |
| Personal Capital | US brokerages only |
| Notion templates | Flexible but no FX historical data, manual everything |
The core issue: no consumer tool assumes you have assets in two currencies that both matter. They treat foreign currency as a "holding" to be converted once. For a couple who genuinely lives in both economies — paying rent in yen, paying student loans in dollars, contributing to both 401(k) and iDeCo — you need both currencies tracked as first-class citizens.
The 4-layer account structure that actually works
Before you open any spreadsheet, get the account taxonomy right. This is the part most people skip and regret.
Layer 1: Liquid cash (by currency)
- USD checking (Chase, BofA, Schwab)
- USD savings / HYSA (Marcus, Ally)
- JPY checking (MUFG, SMBC, Mizuho, Japan Post)
- JPY savings (Rakuten Bank, Sony Bank)
Layer 2: Investment accounts (by tax treatment)
- US taxable brokerage (Fidelity, Vanguard, Schwab)
- US tax-advantaged (401(k), Roth IRA, HSA)
- JP taxable (SBI Securities, Rakuten Securities 特定口座)
- JP tax-advantaged (NISA, iDeCo)
Layer 3: Real assets
- Property (mark-to-market, with currency of location)
- Vehicles (depreciating, often ignored — fine)
Layer 4: Liabilities
- US credit cards, student loans, mortgage
- JP credit cards, 住宅ローン, 奨学金
The rule: every account lives in exactly one currency at the transaction level, and gets converted to a base currency only at the reporting layer. Never store dual-currency balances on a single row. That's how you end up double-counting.
Handling FX rates without going insane
This is where most DIY spreadsheets die. You have two reasonable approaches:
Approach A: Month-end snapshot rate
Every month, pull the USD/JPY rate on the last business day and use that single rate to convert all JPY balances. Simple, reproducible, good enough for 95% of households.
=GOOGLEFINANCE("CURRENCY:USDJPY", "close", EOMONTH(TODAY(),-1))
Or in Excel with a Stocks data type:
=STOCKHISTORY("USDJPY", EOMONTH(TODAY(),-1), EOMONTH(TODAY(),-1), 0, 0, 1)
Approach B: Cost-basis FX tracking
Track what rate you acquired each yen asset at, then compute FX P&L separately from market P&L. This is what sophisticated investors do. It lets you answer: "Did my Japanese stock fund lose money, or did the yen just weaken?"
I recommend Approach A for the first year. Once you have 12 months of data, you can layer B on top.
The gotcha: mid-month transfers
If you wire $10,000 from Schwab to MUFG on the 15th, don't use the month-end rate for that transfer. Record the actual rate your bank gave you (usually worse than mid-market by 1-3%). Wise and Revolut give you near-mid rates; traditional bank wires do not. That spread is real money.
The monthly review ritual (45 minutes, once a month)
Here's the process I run on the first Sunday of every month. Block the time on a shared calendar. No phones.
Checklist
- [ ] Pull balances from all US accounts (aggregator or manual)
- [ ] Pull balances from all JP accounts (Moneytree or manual)
- [ ] Grab month-end USD/JPY rate
- [ ] Enter balances into the snapshot sheet
- [ ] Review auto-calculated USD and JPY totals
- [ ] Check month-over-month delta in both currencies
- [ ] Check FX contribution vs. real contribution to the delta
- [ ] Update the one-line "what changed and why" note
- [ ] Discuss one open question with spouse (5 min)
- [ ] Schedule next month's review
The "what changed and why" note is the single most valuable habit. Example entries from my own log:
2024-03: Net worth up ¥890k MoM, but USD-equivalent flat. Yen weakened from 149 to 151. Real savings: ¥620k. FX: +¥270k paper gain on USD assets.
2024-07: NW down $3,200 USD despite $4,800 in contributions. NISA fund dropped 6%. Nothing to act on — continuing DCA.
You'll be grateful for these notes in year three when you're trying to remember why Q2 2024 looked weird.
A minimal spreadsheet schema
Here's the bare-bones version you can build in an hour. Three sheets:
Sheet 1: accounts
| account_id | name | currency | type | owner |
|---|---|---|---|---|
| US01 | Chase Checking | USD | cash | joint |
| US02 | Fidelity 401k | USD | retirement | spouse_a |
| JP01 | MUFG 普通 | JPY | cash | joint |
| JP02 | SBI NISA | JPY | retirement | spouse_b |
Sheet 2: snapshots
| date | account_id | balance |
|---|---|---|
| 2026-01-31 | US01 | 8420.15 |
| 2026-01-31 | JP01 | 1250000 |
Sheet 3: fx_rates
| date | usdjpy |
|---|---|
| 2026-01-31 | 148.50 |
Then a dashboard sheet with formulas like:
Total USD = SUMIFS(balance, currency, "USD", date, report_date)
+ SUMIFS(balance, currency, "JPY", date, report_date) / USDJPY
Total JPY = SUMIFS(balance, currency, "JPY", date, report_date)
+ SUMIFS(balance, currency, "USD", date, report_date) * USDJPY
This gets you 60% of the value in one afternoon. It will not, however, handle:
- Historical charts in both currencies
- FX-attribution vs contribution-attribution
- Multi-year CAGR in each currency
- Retirement glide-path projections against dual goals
- Spouse-level breakdowns for 確定申告 and US tax filing
- Category rollups (cash vs. invested vs. real estate)
- Conditional formatting for "you're off target"
That's the 40% that took me the next 18 months to figure out.
Common mistakes I made so you don't have to
Mistake 1: Using a single base currency. I started in USD-only. My wife, whose salary and spending were mostly yen, felt disconnected from the numbers. We switched to dual-primary display — every metric shown in both currencies side by side. Engagement doubled.
Mistake 2: Ignoring tax-adjusted values. $100k in a Roth IRA is worth more than $100k in a taxable brokerage, which is worth more than ¥15M in a Japanese 特定口座 (20.315% on gains). For net worth tracking, nominal is fine. For retirement planning, apply a tax haircut.
Mistake 3: Tracking too frequently. I tried weekly for a month. It was noise. Monthly is the right cadence. You're measuring a slow-moving variable, and daily FX chatter will make you do dumb things.
Mistake 4: Not tracking the FX contribution separately. If you don't decompose your month-over-month change into (a) money you added, (b) market returns, and (c) FX moves, you cannot tell whether your plan is working. I had three straight months where our JPY savings rate looked awful — turned out we were saving fine, the yen had just strengthened and compressed our USD-denominated contributions on paper.
Mistake 5: Separate spreadsheets per spouse. We did this for "privacy" reasons early on. It meant reconciliation every month took two hours. Merge them. Use an owner column if you need attribution.
A realistic conversation this enables
Here's what a Sunday review looks like once the system is running. Names changed.
Maya: Okay, January snapshot. USD total is $412,800, JPY total is ¥61.3M.
David: How much of the ¥890k jump was us actually saving?
Maya: Contributions were ¥540k. Market was up ¥180k on the NISA. FX gave us ¥170k because the yen weakened.
David: So real progress is ¥720k. We're on track for the ¥8M annual target.
Maya: Yeah. But I want to talk about the Rakuten savings account. It's sitting at ¥3.2M earning 0.02%. At current inflation that's losing ~2% real per year.
David: Move ¥2M into the NISA growth slot next month?
Maya: Agreed. I'll add it to the action log.
That conversation is impossible without the data structure underneath it. And it took eight minutes.
When to graduate from a spreadsheet to real software
Honest answer: most couples never need to. A well-built spreadsheet handles net worth tracking better than any SaaS tool I've tested for the cross-border case, specifically because:
- You control the currency logic
- You own your data (GDPR, APPI, nothing leaves your machine)
- No monthly subscription
- Formulas are auditable by either spouse
- Works offline on a plane between Narita and SFO
The only reason to graduate is if you cross into business/estate territory: multiple rental properties, a holding company, trust structures. At that point you want an accountant and proper accounting software, not a tracker.
Putting it all together
If you've read this far, you have enough to build a working system yourself. The ingredients are:
- A clean account taxonomy (4 layers, single currency per account)
- A monthly snapshot ritual (45 min, first Sunday)
- A dual-currency reporting layer with FX handled correctly
- A decomposition of MoM change into contribution / market / FX
- A running "what changed and why" log
- Tax-adjusted values for retirement planning, nominal for net worth
Building the Excel file from scratch took me about 40 hours over 18 months of iteration. The version I use now has 14 sheets, handles retirement glide paths in both currencies, does spouse-level breakdowns that map cleanly to both US Form 1040 and Japanese 確定申告 categories, and produces a one-page PDF summary I can send to our financial advisor.
I systematized all of this — the account schema, the FX decomposition formulas, the monthly dashboard, the retirement projections, the tax-lot breakdowns — into a ready-to-use Excel file called the Multi-Currency Net Worth Dashboard for US-Japan Couples. It's what I wish someone had handed me in 2021 instead of me reinventing it over three years.
What's inside:
- Pre-built account schema for typical US-Japan household (customize in 15 min)
- Auto-updating USD/JPY via Excel Stocks data type
- Monthly snapshot sheet with validation rules
- Dual-currency dashboard (USD-primary and JPY-primary views)
- FX vs. contribution vs. market attribution
- Retirement glide-path with dual targets
- Tax-lot tracking for NISA, iDeCo, 401(k), Roth
- Spouse-level rollups for tax filing prep
- 12-month trailing charts in both currencies
- Works in Excel desktop (Mac + Windows) and Excel for web
If you'd rather spend your weekend actually reviewing your finances than building the tool to review your finances, it's $29 — roughly what you'll save on a single bad FX conversion.
View the dashboard on Gumroad →
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