個人事業主 税金予測シミュレーター

Overview

A web app that allows sole proprietors and freelancers to estimate in real-time their annual tax and social insurance burden for 2026 based on current sales and expenses. Rather than creating a final tax return, it is a tax preparation tool designed to help users understand early on: "If the current pace continues through year-end, how much might I need to pay, and how much should I set aside each month starting now?"

Main Features

• Step-by-step input — Enter basic information, sales, expenses, deductions, and other items in order. Usable even without detailed tax knowledge
• Toggle between actual results and forecasts — Forecast year-end based on current results, or support detailed monthly input
• Itemized breakdown of 7 tax categories — Income tax, reconstruction special income tax, resident tax, individual business tax, consumption tax, National Health Insurance, and National Pension
• Visualization via graphs and tables — Display sales, expenses, profit, and savings targets for months 1–12, distinguishing between actual and forecast
• Auto-calculate monthly preparation targets — Deduct secured funds from annual burden and present the monthly preparation amount for remaining periods
• Compare 4 scenarios — Review the effects of additional expenses, small business mutual aid, and iDeCo side by side
• Tax payment timeline — Display typical tax payment dates
• Expandable calculation basis display — Confirm "why this amount?" and calculation basis / official sources for each tax category
• Sample data — Test calculations with fictional samples for annual sales of ¥3M, ¥6M, ¥10M, and ¥15M
• Privacy protection — No need to enter name, address, or My Number. Data is saved only in browser localStorage

Supported Input Items

Business & Filing — Business start date/month, prefecture, filing method (blue or white), choice of blue filing special deduction

Sales & Expenses — Cumulative amount input or detailed monthly input. Expenses support 16 categories

Income Deductions — Basic deduction auto-determined. Directly input social insurance premiums, small business mutual aid, iDeCo, life insurance premiums, medical expenses, spousal deduction, dependent deduction, etc.

Consumption Tax — Support for exemption, standard taxation, simplified taxation, and 2% special measure

Technical Specifications

• Built with Next.js 16.3, React 19.2, TypeScript
• Graph display with Recharts
• Tax calculations verified by 84 unit tests using Vitest
• Responsive design for PC, tablet, and smartphone
• Accessibility considerations including keyboard operation, visible focus, and aria attributes
• Published on Firebase Hosting

Target Users

Those who recently started a business and want visibility into annual burden; blue filing and white filing business operators; company employees with business income from side work; those who want to estimate consumption tax burden after registering for the invoice system; and others.