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
Karl's Mortgage Calculator Pro
This is the Pro version of Karl's Mortgage Calculator
$1.99 · In-app purchases
PRICE
$1.99
In-app purchases
BUSINESS MODEL
Paid
No overall Grossing estimate available
RATING
★ 4.8
10 ratings · Top 5%
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
Make this idea yours
A product spec, screen plans, and build steps for your coding agent.
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Combine features and design from your favorite apps.
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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 included17 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 Karl's Mortgage Calculator Pro: map the core Finance workflow, choose the smallest differentiated feature set, define the data and monetization boundaries, then prototype the riskiest user journey first.
Preview ready. Verify your email to reveal and copy the full prompt.
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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
Include more customizable features and settings.
Assumption to check: Limited customization options for advanced users
Validate with people who use this workflow before building.
Hypothesis 2
Add functionality to export results as PDF or CSV.
Assumption to check: Lack of export options for results
Validate with people who use this workflow before building.
Hypothesis 3
Integrate with APIs for real-time mortgage rates and bank data.
Assumption to check: No integration with financial institutions for real-time data
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
This is the Pro version of Karl's Mortgage Calculator. The only change from the free version is that the adverts have been removed. Calculate mortgage payments given principal, interest and term. Reverse calculate any one variable given the other three. Enter a down payment amount or percentage and let the calculator show how large a mortgage you require. See how the monthly payment changes when you factor in additional monthly or annual loan costs such as PMI, HOA, taxes and insurance. The calculator supports Fixed and Adjustable Rate Mortgages (ARM) loans with up to five changes in interest rates. Interest-only mortgages loans are also supported. You can pay off your mortgage early with extra payments. Add one-off, monthly and annual extra payments. Choose up to six additional extra payment amounts and payment periods and the calculator will show how much money you can save on the loan. Choose from weekly to annual loan payments and adjust the compounding period from daily through annual periods. The calculator supports Canadian mortgages along with UK annual rest mortgages. Multiple rounding options on loan calculations are also available. Choose between no rounding, round half up, half down, to even, to ceiling and to floor. The calculator also supports the input and display of multiple currencies.
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
React Navigation
Yup
@react-native-community/picker
react-native-paper
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.
