Feature spec
Goal: Send a clear invoice after finishing a job.
- Add a client, line items, a due date, and payment instructions.
- Preview the total before exporting a PDF.
- Keep an editable draft if export fails.
App Overview
Amortization Calculator
The Amortization Calculator app is a must-have for people with a mortgage, auto loan, student loan, or any other loan that requires monthly payments.
$0.99 · In-app purchases
PRICE
$0.99
In-app purchases
BUSINESS MODEL
Paid
No overall Grossing estimate available
RATING
N/A
No ratings yet
Evidence scope: No current overall US chart position is available. Category-chart positions shown in discovery are a separate scope. Revenue is a directional model estimate, not verified earnings.
Build your own version
Build Kit
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A product spec, screen plans, and build steps for your coding agent.
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What should your app do differently? Your note travels with the kit so your coding agent builds around your idea.
What's included15 files
A product spec, phased build plan, differentiation notes, and screen references.
- START_HERE
- AGENTS
- EVIDENCE
- SPEC
- BUILD_PLAN
- DIFFERENTIATION
- skills/
- screens/
Preview a sample Build Kit No email needed
Illustrative example: an invoice app for independent contractors. Your kit is tailored to the app you choose; this is not its generated content.
Screen plan
Invoice editor: Client at the top, editable line items in the middle, total and preview action at the bottom.
States: Empty draft, validation errors beside each field, exporting, and a retry action that preserves the draft.
Differentiate and validate
Hypothesis: Contractors need faster repeat invoices more than more templates.
First experiment: Test duplicating a previous job with five contractors. Watch where they hesitate before expanding the feature set.
The full kit adds a phased build plan, evidence notes, tool prompts, and implementation guidance. It is a plan for your coding agent, not a finished app.
Usage: Mechanics are fair game; never reuse the original name, branding, assets, or verbatim copy.
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Free quick prompt For a smaller starting point
Quick prompt
Good fit for mobile-first iOS and Android prototypes with managed app scaffolding.
// RORK SYSTEM PROMPT
Rork plan preview for Amortization Calculator: map the core Finance workflow, choose the smallest differentiated feature set, define the data and monetization boundaries, then prototype the riskiest user journey first.
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Opportunities
AI-GENERATED OPPORTUNITY HYPOTHESES
These are generated suggestions, not verified review quotations. Check current App Store reviews and speak with users before treating a possible gap as a requirement.
Hypothesis 1
Implement loading indicators and success/error messages.
Assumption to check: Limited user feedback during calculations
Validate with people who use this workflow before building.
Hypothesis 2
Add social sharing functionality.
Assumption to check: No sharing options for calculations
Validate with people who use this workflow before building.
Hypothesis 3
Introduce a History screen to save and display past calculations.
Assumption to check: Lack of historical data tracking
Validate with people who use this workflow before building.
Hypothesis 4
Allow users to save preferred settings like currency and default loan terms.
Assumption to check: No customization options for users
Validate with people who use this workflow before building.
App evidence
Original App Store material. Ratings and screenshots describe the existing app, not proof of demand for your version.
App Store description
The Amortization Calculator app is a must-have for people with a mortgage, auto loan, student loan, or any other loan that requires monthly payments. The Amortization Calculator starts by calculating your monthly payment based on the loan amount, interest rate, and length of your loan. Then it creates an amortization schedule that breaks down how much of each monthly payment goes towards interest, how much goes towards paying down the loan, and the current loan balance. The amortization schedule will show you that as times goes by, less of your monthly payments will go towards paying interest and more of your monthly payments will go towards paying down the loan. You can substantially reduce the number of monthly payments and total interest cost of your loan by either paying a fixed extra amount every month, or paying a large one-time lump sum. This app also includes an Extra Monthly Loan Payment Calculator and a Lump Sum Loan Calculator so you can calculate your potential savings. The app was inspired by the popular Grinebiter.com Amortization Calculator web application. We took what we learned from our web visitors to make this very useful iOS Apple app. The app is fast, intuitive, and easy to use. It is a standalone app, which means that it will work perfectly fine in places with little or no Internet access. Additionally, the Amortization Calculator app does not have any subscription fees, in-app purchases, tracking, or ads. In other words, you can enjoy full and complete access to the app with no surprises or extra costs. We hope that the Amortization Calculator app will become one of your favorite apps to help with your financial health.
Written user reviews are not shown here. Check current App Store reviews >
Screenshots
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Build scope
Plan a focused first version with your coding agent. These are planning assumptions, not a delivery guarantee.
Define the first version
Choose one audience and one core workflow. Use the kit to agree on its screens, data, and acceptance criteria before building.
Set boundaries
Decide which secondary features, integrations, and platform support can wait. Your version does not need to reproduce everything in the original.
Check the hard parts
Validate external services, specialist technology, data access, and ongoing costs for your chosen scope.
SUGGESTED BUILD STACK
React Native
Expo
Axios
React Navigation
React Native Reanimated
React Native Async Storage
BUILD KIT DIFFICULTY
Starter
From the Build Kit assessment.
A reliable timeline needs an agreed scope and a technical check. Ask your agent to estimate the phases in BUILD_PLAN.md after that review.
