Sea freight from 1688 to Vietnam typically takes 25 to 45 days, and that gap does not shrink just because you are running a campaign. Most stockouts on imported goods happen not because a seller forgot to reorder, but because they set the reorder trigger too low and did not keep a meaningful buffer for when sales spike or freight runs late. Safety stock is that buffer, and sizing it correctly is what keeps you selling through the wait.
What safety stock is and what it is not
Safety stock is the quantity you hold above your expected selling needs. It is not inventory to sell. It is inventory to absorb two things that routinely deviate from plan: demand running faster than your average, and stock arriving later than your usual lead time.
For goods sourced domestically, a shop can place a top-up order and receive it within days. With 1688 imports, the gap between placing an order and receiving it is long enough that almost anything can go wrong in between. Sea freight alone runs roughly 18 to 30 days in transit, plus time at the consolidation warehouse, customs clearance, and domestic delivery at the Vietnam end. Air freight compresses this to around 8 to 15 days, but at a cost that changes the margin math significantly.
Running out of stock mid-campaign is not just a lost-revenue problem. Some platforms track your fulfillment record and cancellation rate to rank listings. Cancellations from stockouts hurt that score and affect your visibility well beyond the days you were out.
The two variables that set your safety stock level
A basic safety stock calculation has two components.
Demand variability. Not every day sells at the same pace. A week with heavy ad spend can sell double a quiet week. A peak season can triple it. Measure this spread: take your highest daily sales in a recent one-month window and subtract your average daily sales. That gap is how much demand can overshoot your plan on any given stretch.
Lead time variability. Inbound from 1688 does not always hit its schedule. Goods get held at the consolidation warehouse, containers get rolled, customs slows around major holidays, especially in the weeks before Tet or during the 11/11 and 12/12 shopping events when the entire supply chain is congested. Measure this too: take the longest lead time you have actually experienced and subtract your average lead time.
A workable minimum safety stock is approximately: (demand variability per day) x (average lead time) + (average daily sales) x (lead time variability in days).
A spreadsheet is enough to run this. No specialized software required.
A worked example
Take a phone accessory SKU as an example:
- Average daily sales: 20 units
- Peak daily sales in the last month: 32 units (heavy campaign day)
- Demand variability: 32 - 20 = 12 units per day
- Average sea freight lead time: 35 days
- Longest lead time experienced: 42 days (customs delay)
- Lead time variability: 42 - 35 = 7 days
Safety stock = (12 x 35) + (20 x 7) = 420 + 140 = 560 units
So beyond the stock expected to cover normal selling pace, this shop should hold an extra 560 units as a buffer. If the following week sells hard or the next shipment arrives a few days late, the warehouse does not hit zero before the new batch lands.
This number changes per SKU. A slow, steady seller needs far less buffer. A SKU with large swings tied to campaigns needs more.
Reorder point and safety stock are two different numbers
These are easy to conflate. The reorder point is the inventory level at which you place the next purchase order. It includes the quantity that will sell during the entire lead time, plus safety stock.
Reorder point = (average daily sales x average lead time) + safety stock.
From the example: (20 x 35) + 560 = 700 + 560 = 1,260 units.
When your on-hand count drops to 1,260 units, that is when you place the next order. Waiting until you have 200 or 300 units left is already too late with a 35-day lead time.
A common failure pattern: the safety stock number is set correctly, but the trigger to place a new order is set far too low. The buffer exists in theory, but the reorder never fires early enough to use it.
Adjusting for peak seasons and major campaigns
Safety stock is not a fixed annual number. Before campaigns like 11/11, 12/12, and Tet, daily sales can spike sharply in a short window while lead times simultaneously stretch because the entire supply chain is under pressure. Both variables move in the wrong direction at the same time.
Practical adjustments:
- Recalculate safety stock for peak season separately, using last year's peak-period sales data rather than your current average.
- Place orders at least two weeks earlier than your usual schedule before a peak event. Not because you will run out sooner, but because consolidation warehouses and freight carriers are also overloaded during those periods, so your actual lead time will be longer than normal.
- Do not cut safety stock immediately after a peak. For a few weeks after 11/11 or 12/12, many shops carry higher-than-normal stock because they over-ordered but sales have slowed. Keep the buffer elevated until daily velocity has genuinely returned to baseline, not just in theory.
Holding too much is also a real cost
More safety stock is not always better. Every unit sitting in the warehouse is capital not doing anything. With 1688 goods where lead times are long by nature, carrying excess buffer means money locked in inventory instead of funding a new SKU or ad spend.
A reasonable safety stock is sized to absorb the real measured variability, not the variability you imagine based on anxiety. If you have not actually measured your demand swings and lead time variance, start there before you pick a buffer number.
A signal that your safety stock is set too high: your total on-hand inventory consistently stays above two to three full lead times of selling pace, with no peak season approaching. That is a sign to revisit the number.
Different product types call for different approaches
Not every SKU needs the same formula weight.
- Seasonal or trend goods carry a real risk on the downside. If you build a large safety stock buffer for a peak period and demand drops sharply after the season ends, that buffer becomes slow-moving stock. Size the peak-season safety stock separately and do not carry it into the off-season calculation.
- Goods from small or capacity-constrained suppliers on 1688: some factories run short unexpectedly, and you may face a wait much longer than normal freight lead time. For these, factor supplier-side supply risk into the buffer, not just shipping variance.
- Heavy or bulky goods: freight cost is a large share of landed cost on these. Carrying excess safety stock ties up both warehouse space and capital disproportionately. Account for the holding cost when deciding how large a buffer to keep on high-weight SKUs.
Bottom line
Correct safety stock is not about keeping as much as possible in the warehouse. It is about holding exactly enough to absorb real, measured demand and lead time variability across the long replenishment cycle that 1688 imports require. Measure the actual variability, set your reorder point separately from your safety stock level, and recalibrate before major selling events. Those three steps prevent most mid-campaign stockouts on imported goods.