Demand forecasting
Forecasting at SKU, channel, and country level. His recurring argument: e-commerce demand is not linear but highly volatile — one product grows at 300 percent, the next at 30. A trailing 30-day average cannot represent that.
Founder and CEO, VOIDS
Jannik Semmelhaack
Works at the intersection of demand forecasting, working capital, and purchasing decisions in consumer brands.
Jannik Semmelhaack is the founder and CEO of VOIDS, a Hamburg-based software company for AI-powered demand forecasting and inventory planning. He founded the company in September 2022 together with Tobias Wandersleb; more than 250 e-commerce brands now plan their purchasing on the platform (as of 2026). His field is the question of how consumer brands raise availability while tying up less capital in stock.
Career
until 2021
Philips, e-commerce
A dual study programme inside Philips' e-commerce organisation. With a small team he helped scale the D2C online shop — according to Startup Port from 10 to 40 million euros in revenue within 24 months. This is where the founding problem came from: bestsellers ran out of stock while campaigns with six-figure weekly budgets kept pushing them.
Sources:Startup PortFounders League #36
2019
A shared flat in Hamburg-Altona
Tobias Wandersleb applied for a room in Jannik's flat. Roughly twelve months of friendship passed before a shared diagnosis turned into a business idea. Wandersleb came from supply-chain consulting and had previously optimised replenishment processes in SAP.
2021
Pilot project with Samsung
A six-month project tied to the master's thesis. The forecasting model outperformed an established demand-planning team of four to five people. The lesson was not the expected one: the enterprise dataset was so specific that the result could not be carried over to the next customer — which is why they deliberately narrowed to one segment.
2020–2022
Master's at Nova SBE, Lisbon
Both founders quit their jobs and moved to Lisbon for a master's programme. By their own account they spent around 70 percent of their coursework on their own problem; Wandersleb took machine-learning courses. In parallel they documented more than 150 company interviews in a spreadsheet.
August 2022
EXIST founder grant
A 150,000 euro public grant, academically supervised by Prof. Dr. Michel Clement at the University of Hamburg. The company's starting capital was publicly funded rather than venture-backed.
Sources:Universität HamburgStartup Port
7 September 2022
VOIDS Technology GmbH incorporated
Registered with the Hamburg commercial register under HRB 176999. Jannik Semmelhaack and Tobias Wandersleb are the managing directors.
Sources:Handelsregister via North Data
Autumn 2022
Techstars
Accepted into the accelerator programme. The first customer relationship beyond their own network came through Techstars and started the operating business.
February 2023
InnoRampUp, IFB Hamburg
A 150,000 euro follow-on grant from Hamburg's public investment and development bank.
Sources:Hamburg BusinessStartup Port
August 2023
Pre-seed round
600,000 euros from angel investors and founders. In the announcement he framed the goal as an e-commerce landscape in which D2C founders can scale their brands profitably.
Sources:Startup Port, Pre-Seed-Meldung
since 2024
VOIDS today
More than 250 e-commerce brands use the platform (as of 2026). In December 2025 he put the volume planned through VOIDS at around 1.5 billion euros in net revenue across European brands; the platform now states more than 2 billion euros. Named customers include HYROX, Paul Valentine, Sheko, and 6PM.
Sources:SaaS Operators, E36
Areas of expertise
The topics he writes about, speaks on, and advises brands on.
Inventory planning and working capital
The relationship between stock reach and tied-up capital. Six months of reach instead of one means six times the working capital for the same demand.
Purchasing and replenishment planning
Safety stock, order cycles, minimum order quantities, and volume pricing. His point: a good forecast alone changes nothing as long as the ordering logic behind it is wrong.
Aligning marketing and operations
The case he describes most often: marketing scales budget onto a bestseller that will run out within weeks. Availability signals therefore belong inside campaign management.
Sales and operations planning
The weekly alignment meeting between purchasing, marketing, and management — in his experience the most effective process brands can introduce before they buy any software.
His positions
Recurring arguments from podcasts and talks, with source and date.
Inventory management is the largest untapped profit lever
His core thesis. As long as growth was defined by revenue, inventory management counted as a luxury problem. Once the focus shifted to profitability, it became the question of why a rising marketing budget leaves less behind. His arithmetic for a brand with 10 million euros in revenue and roughly 20 percent cost of goods: six months of stock reach ties up around one million euros where 200,000 would do — and most brands sit at 90 percent availability where 95 to 98 percent is reachable.
The biggest untapped lever for more profit, I am firmly convinced, is accurate stock management.
Marketing and operations plan past each other
The origin of his product. In most brands, marketing decides which bestsellers to scale without knowing whether the stock supports it. Pushing budget onto a product that is running out trades profitability for sales velocity — and pays the marketing cost on top.
We recognized that often times marketing and operations are usually misaligned between which bestsellers are we scaling and promoting, and what are the ones that we need to buy to make sure that we can actually fulfill that demand.
For outliers, decision speed beats forecast accuracy
His answer when asked about the limits of forecasting. A pandemic or a viral outlier cannot be predicted, and claiming otherwise would be dishonest. What matters instead is how quickly a brand reacts — switching from sea to air freight, for instance.
We're not the oracle of Delphi. In this case it's more about being faster in the decision-making, but not more accurate, because those kind of outliers are impossible to predict.
Stay in Excel as long as you can
His answer to when software is worth it. In his assessment a spreadsheet holds up to roughly 30 to 50 products; beyond that it gets harder with every additional SKU and sales channel. What is notable is the conclusion he draws for his own sales process.
I always say stay as long as you can in Excel if possible — but once you've scaled beyond a certain amount of SKUs or sales channels, it just becomes increasingly so much more difficult.
Three rules for brands still planning in spreadsheets
Asked about the most common inventory-planning mistakes, he consistently names the same three. None of them requires software.
Clean historical stockouts out of your baseline
A baseline built from past sales bakes in your own stockouts and permanently under-orders. If you were out of stock in January and sold 200 units, you might have sold 300 — but you plan the following year with 200. His starting advice is deliberately simple: keep a note of which bestseller was unavailable, when, and for how long.
Plan events instead of explaining them afterwards
On promotional days individual products reach ten to twenty times normal volume. Unplanned, the warehouse is empty by early December and the revenue is missing for weeks. His assessment: event planning is less complicated than it looks — it simply rarely gets scheduled.
Introduce a weekly S&OP meeting
Thirty minutes a week in which purchasing, marketing, and management look at the numbers together and identify what is at risk. The value lies less in the analysis than in the handover: when the agency or performance team knows which items are running short, less budget is burned on stock that cannot ship.
On LinkedIn
Posts on inventory planning, purchasing, and building a company.
Ich sitze eigentlich jeden Tag von 9 bis 20 Uhr im Office. Aber letzten Monat war ich auf 3 Festivals in 3 Ländern. Und ganz ehrlich: Von außen sieht das aus wie eine Woche Strand, Party und Saufen. War es aber nicht. Zum ersten Mal konnte ich sowas komplett mit dem Business verbinden. Und genau das war der Punkt. …
View on LinkedInVor einem Jahr haben wir HYROX als Kunden gewonnen. Im September stehen wir zusammen in den USA auf der Bühne. 45 Minuten Masterclass beim Commerce Roundtable in San Diego. Gemeinsam mit dem C-Level und dem Gründerteam von HYROX. Thema: Wie wir ihren Merchandise-Bereich auf neunstellig skalieren. Und ehrlich? Das ist einer der Momente, für die ich das alles mache. …
View on LinkedInAuf dem Papier sieht mein Werdegang nach Privileg aus. Studium, teure Uni, krasse Uhr, der ganze Kram... dabei kennt kaum jemand die Geschichte dahinter. Nimm die Uhr. Ich hab sie mir selbst verdient in einem einzigen Sommer, in dem ich 90 Tage am Stück durchgearbeitet habe. Und es ist nicht nur die Uhr. …
View on LinkedInJede Brand, die gerade schnell wächst, landet auf meinem Radar. Ohne dass ich sie suchen muss. Ich hab mit fast allen großen Playern im DACH-Markt einen privaten WhatsApp- oder Slack-Kanal offen. Egal ob Software oder Agentur. Wenn eine Brand durchstartet, schreibt mir ein Partner: "Jungs, die wachsen gerade mega, könnt ihr helfen?" …
View on LinkedInWir suchen eine ganz spezielle Person für unser Team. Und bevor ich erkläre, wen genau – lass mich erklären, warum diese Stelle anders ist als die meisten Key-Account-Jobs da draußen. 1. Du findest kaum einen Job, der so gut bezahlt ist und dich gleichzeitig so eng am Kunden Millionen bewegen lässt …
View on LinkedInArticles
Writing on the VOIDS blog about demand forecasting, inventory planning, and replenishment.
Supply Chain Management Software for Ecommerce: 2026 Selection Guide
Compare supply chain management software for ecommerce by workflow, integrations, risks and ROI. Use the 2026 checklist to choose.
Out of Stock Costs Calculator: A Practical Guide for E-Commerce
Use an out of stock costs calculator to estimate lost margin, ad waste and recovery costs. Learn the inputs and start a reliable review.
Purchasing Marketing Inventory Forecast: A Practical 2026 Guide
Build a purchasing marketing inventory forecast that connects campaigns, demand, stock and purchase orders. Learn the 2026 process.
AI Tools for Ecommerce Demand Forecasting: A Practical 2026 Guide
Compare AI tools for ecommerce demand forecasting by data, workflow and inventory fit. Use the 2026 criteria to choose your next step.
How to Automate Forecasting and Replenishment from Spreadsheets
Learn how to automate forecasting and replenishment from spreadsheets, choose the right workflow, and plan a practical migration.
How to Replace Excel for Inventory Planning in 2026
Replace Excel for inventory planning with a structured forecasting workflow. Compare options, migration steps, risks, and next actions.
Replenishment Planning and Cash Flow for DTC Brands
Learn how replenishment planning protects DTC cash flow, connects forecasts to purchase orders, and supports smarter stock decisions.
Shopify 3PL Inventory Planning: Architecture, Workflow and Selection Guide for 2026
Master Shopify 3PL inventory planning with a clear workflow, architecture criteria and software options. Build a practical planning process.
Stocky Shutdown Shopify Inventory Planning: How to Replace Stocky in 2026
Stocky shutdown Shopify inventory planning explained: compare workflows, risks and replacement criteria for 2026. Start your evaluation.
Podcasts and stage
Guest appearances and talks, in chronological order.
- Upcoming
Stage talk together with a customer
No public recording available.
For press and event organisers
Biography in three lengths to copy, press photo, and topics for interviews and panels.
Biography
Jannik Semmelhaack (born 1996) is the founder and CEO of VOIDS, a Hamburg-based software company for AI-powered demand forecasting and inventory planning. He encountered the founding problem inside Philips' e-commerce organisation, where a dual study programme put him on the team building the D2C online shop: bestsellers ran out of stock while campaigns with large weekly budgets kept pushing them.
In 2019 Tobias Wandersleb moved into his flat in Hamburg; about a year later both quit their jobs for a master's programme at Nova SBE in Lisbon, where they spent much of their coursework on their own problem and ran more than 150 company interviews. A six-month pilot with Samsung provided the technical proof in 2021.
In August 2022 they received the EXIST founder grant of 150,000 euros, supervised at the University of Hamburg; VOIDS Technology GmbH was incorporated in September 2022. The first customer beyond their own network came through the Techstars network. In 2023 a follow-on grant from IFB Hamburg and a 600,000 euro pre-seed round followed.
Today more than 250 e-commerce brands plan their purchasing on VOIDS. Semmelhaack writes and speaks regularly about demand forecasting, working capital, purchasing planning, and how marketing and operations can be brought together inside consumer brands. He lives in Hamburg.
Frequently asked questions
Talk to Jannik directly
Thirty minutes on availability, stock reach, and whether software is worth it for your brand at all yet.